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Universal Disk Format
Universal Disk Format
Universal Disk Format
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UDF
Developer(s)ISO/IEC, Ecma International, OSTA
Full nameUniversal Disk Format
Introduced1995; 30 years ago (1995)
Partition IDsNot assigned but suggested:[1]
0x07 (MBR)
EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 (GPT)
Limits
Max volume size2 TiB (with 512-byte sectors), 8 TiB (with 2 KiB sectors, like most optical discs), 16 TiB (with 4 KiB sectors)[a][2]
Max file size16 EiB
Max filename length255 bytes (path 1023 bytes[b])
Allowed filename
characters		Any 16-bit Unicode Code point excluding U+FEFF and U+FFFE
Features
Dates recordedcreation, archive, modification (mtime), attribute modification (ctime), access (atime)
Date range
24:00:00.000, 1 January 1 (UTC)  – 23:59:59.999, 31 December 9999 (UTC) [3]
Date resolutionMicrosecond
ForksYes
AttributesVarious
File system
permissions		POSIX
Transparent
compression		No
Other
Supported
operating systems		Various

Universal Disk Format (UDF) is an open, vendor-neutral file system for computer data storage for a broad range of media. In practice, it has been most widely used for DVDs and newer optical disc formats, supplanting ISO 9660. Due to its design, it is very well suited to incremental updates on both write-once and re-writable optical media. UDF was developed and maintained by the Optical Storage Technology Association (OSTA).

In engineering terms, Universal Disk Format is a profile of the specifications known as ISO/IEC 13346 and ECMA-167.[3]

Usage

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Normally, authoring software will master a UDF file system in a batch process and write it to optical media in a single pass. But when packet writing to rewritable media, such as CD-RW, UDF allows files to be created, deleted and changed on-disc just as a general-purpose filesystem would on removable media like floppy disks and flash drives. This is also possible on write-once media, such as CD-R, but in that case the space occupied by the deleted files cannot be reclaimed (and instead becomes inaccessible).

Multi-session mastering is also possible in UDF, though some implementations may be unable to read disks with multiple sessions.[c]

History

[edit]
Optical discs
General
Optical media types
Standards
See also

The Optical Storage Technology Association standardized the UDF file system to form a common file system for all optical media: both for read-only media and for re-writable optical media. When first standardized, the UDF file system aimed to replace ISO 9660, allowing support for both read-only and writable media. After the release of the first version of UDF, the DVD Consortium adopted it as the official file system for DVD-Video and DVD-Audio.[4]

UDF shares the basic volume descriptor format with ISO 9660. A "UDF Bridge" format is defined since 1.50 so that a disc can also contain a ISO 9660 file system making references to files on the UDF part.[5]

Revisions

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Multiple revisions of UDF have been released:[4][6]

  • Revision 1.00[d] (24 October 1995). Original Release.
  • Revision 1.01[e] (3 November 1995). Added DVD Appendix and made a few minor changes.
  • Revision 1.02[7] (30 August 1996). This format is used by DVD-Video discs.
  • Revision 1.50[8] (4 February 1997). Added support for CD-R/W Packet Writing and (virtual) rewritability on CD-R/DVD-R media by introducing the Virtual Allocation Table (VAT) structure. Added sparing tables for defect management on rewritable media such as CD-RW, and DVD-RW and DVD+RW. Add UDF bridge.
  • Revision 2.00[9] (3 April 1998). Added support for Stream Files, Access Control lists, Power Calibration, real-time files (for DVD recording) and simplified directory management. VAT support was extended.
  • Revision 2.01[10] (15 March 2000) is mainly a bugfix release to UDF 2.00. Many of the UDF standard's ambiguities were resolved in version 2.01.
  • Revision 2.50[11] (30 April 2003). Added the Metadata Partition facilitating metadata clustering, easier crash recovery and optional duplication of file system information: All metadata like nodes and directory contents are written on a separate partition which can optionally be mirrored. This format is used by some versions of Blu-rays and most HD-DVD discs.
  • Revision 2.60[12] (1 March 2005). Added Pseudo OverWrite method for drives supporting pseudo overwrite capability on sequentially recordable media. Has read-only compatibility with UDF 2.50 implementations.: 10  (Some Blu-rays use this format.)

UDF Revisions are internally encoded as binary-coded decimals; Revision 2.60, for example, is represented as 0x0260.[12]: 23  In addition to declaring its own revision, compatibility for each volume is defined by the minimum read and minimum write revisions, each signalling the requirements for these operations to be possible for every structure on this image. A "maximum write" revision additionally records the highest UDF support level of all the implementations that has written to this image.[12]: 34  For example, a UDF 2.01 volume that does not use Stream Files (introduced in UDF 2.00) but uses VAT (UDF 1.50) created by a UDF 2.60-capable implementation may have the revision declared as 0x0201, the minimum read revision set to 0x0150, the minimum write to 0x0150, and the maximum write to 0x0260.

Uses

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While UDF was primarily developed for optical discs, it is also able to operate on other media such as hard disk drives and flash storage.[13]

UDF was for well over a decade the only open-specification and cross-platform-supported file system without a file size limitation of 4 GiB (roughly 4.3 GB), until the open-sourcing of exFAT in 2019.[f][g] "Cross-platform" here means supported by all three majorly used operating systems, Windows and Mac OS and Linux.[14]

On Windows, formatting flash storage devices as UDF can not be accomplished through the graphical formatting widget, only through the command prompt.[15]

Specifications

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The UDF standard defines three file system variations, called "builds". These are:

  • Plain (Random Read/Write Access). This is the original format supported in all UDF revisions
  • Virtual Allocation Table, also known as VAT (Incremental Writing). Used specifically for writing to write-once media
  • Spared (Limited Random Write Access). Used specifically for writing to rewritable media

Plain build

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Introduced in the first version of the standard, this format can be used on any type of disk that allows random read/write access, such as hard disks, DVD+RW and DVD-RAM media. Metadata (up to v2.50) and file data is addressed more or less directly. In writing to such a disk in this format, any physical block on the disk may be chosen for allocation of new or updated files.

Since this is the basic format, practically any operating system or file system driver claiming support for UDF should be able to read this format.

VAT build

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Write-once media such as DVD-R and CD-R have limitations when being written to, in that each physical block can only be written to once, and the writing must happen incrementally. Thus the plain build of UDF can only be written to CD-Rs by pre-mastering the data and then writing all data in one piece to the media, similar to the way an ISO 9660 file system gets written to CD media.

To enable a CD-R to be used virtually like a hard disk, whereby the user can add and modify files on a CD-R at will (so-called "drive letter access" on Windows), OSTA added the VAT build to the UDF standard in its revision 1.5. The VAT is an additional structure on the disc that allows packet writing; that is, remapping physical blocks when files or other data on the disc are modified or deleted. For write-once media, the entire disc is virtualized, making the write-once nature transparent for the user; the disc can be treated the same way one would treat a rewritable disc.

The write-once nature of CD-R or DVD-R media means that when a file is deleted on the disc, the file's data still remains on the disc. It does not appear in the directory any more, but it still occupies the original space where it was stored. Eventually, after using this scheme for some time, the disc will be full, as free space cannot be recovered by deleting files. Special tools can be used to access the previous state of the disc (the state before the delete occurred), making recovery possible.

Not all drives fully implement version 1.5 or higher of the UDF, and some may therefore be unable to handle VAT builds.

Spared (RW) build

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Rewriteable media such as DVD-RW and CD-RW have fewer limitations than DVD-R and CD-R media. Sectors can be rewritten at random (though in packets at a time). These media can be erased entirely at any time, making the disc blank again, ready for writing a new UDF or other file system (e.g., ISO 9660 or CD Audio) to it. However, sectors of -RW media may "wear out" after a while, meaning that their data becomes unreliable, through having been rewritten too often (typically after a few hundred rewrites, with CD-RW).

The plain and VAT builds of the UDF format can be used on rewriteable media, with some limitations. If the plain build is used on a -RW media, file-system level modification of the data must not be allowed, as this would quickly wear out often-used sectors on the disc (such as those for directory and block allocation data), which would then go unnoticed and lead to data loss. To allow modification of files on the disc, rewriteable discs can be used like -R media using the VAT build. This ensures that all blocks get written only once (successively), ensuring that there are no blocks that get rewritten more often than others. This way, a RW disc can be erased and reused many times before it should become unreliable. However, it will eventually become unreliable with no easy way of detecting it. When using the VAT build, CD-RW/DVD-RW media effectively appears as CD-R or DVD+/−R media to the computer. However, the media may be erased again at any time.

The spared build was added in revision 1.5 to address the particularities of rewriteable media. This build adds an extra Sparing Table in order to manage the defects that will eventually occur on parts of the disc that have been rewritten too many times. This table keeps track of worn-out sectors and remaps them to working ones. UDF defect management does not apply to systems that already implement another form of defect management, such as Mount Rainier (MRW) for optical discs, or a disk controller for a hard drive.

The tools and drives that do not fully support revision 1.5 of UDF will ignore the sparing table, which would lead them to read the outdated worn-out sectors, leading to retrieval of corrupted data.

An overhead that is spread over the entire disc reserves a portion of the data storage space, limiting the usable capacity of a CD-RW with e.g. 650 MB of original capacity to around 500 MB.[16]

Character set

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The UDF specifications[4] allow only one Character Set OSTA CS0, which can store any Unicode Code point excluding U+FEFF and U+FFFE. Additional character sets defined in ECMA-167 are not used.[3]: 7.2 

Since Errata DCN-5157, the range of code points was expanded to all code points from Unicode 4.0 (or any newer or older version), which includes Plane 1–16 characters such as Emoji. DCN-5157 also recommends normalizing the strings to Normalization Form C.[17]

The OSTA CS0 character set stores a 16-bit Unicode string "compressed" into 8-bit or 16-bit units, preceded by a single-byte "compID" tag to indicate the compression type. The 8-bit storage is functionally equivalent to ISO-8859-1, and the 16-bit storage is UTF-16 in big endian. 8-bit-per-character file names save space because they only require half the space per character, so they should be used if the file name contains no special characters that can not be represented with 8 bits only.[18]

The reference algorithm neither checks for forbidden code points nor interprets surrogate pairs, so like NTFS the string may be malformed.[4]: 2.1.2, 6.4  (No specific form of storage is specified by DCN-5157, but UTF-16BE is the only well-known method for storing all of Unicode while being mostly backward compatible with UCS-2.)[17]

Tools

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Various tools have been developed to work with UDF file systems. For Linux, the udftools package contains several tools that can be used directly as a command-line interface, but can also be called by other applications.

The mkudffs tool creates a new UDF file system on a partition or a disk image file. The tool is symbolically linked from mkfs.udf for consistency with other mkfs tools.

udfinfo shows details of an UDF file system such as the volume label, version, counts of files and directories, and the status of the write protection flags. The volume label can be changed using the udflabel tool.

The write protection flag can "seal" a file system once its authoring is finished and no more changes to it are intended. For toggling the write protection setting which makes a file system read-only when activated, a tool called udftune was under development, but has not yet been included in the udftools package as of April 2025, following that its maintainer Pali Rohár has reportedly lost access to his GitHub account.[19]

wrudf allows the user to modify an UDF file system interactively. For example, files can be copied and removed, and directories can be created and removed.

pktsetup implements packet writing through device files named /dev/pktcdvd0 and counting up. These device files are separate from the usual device files of optical drives named /dev/sr0 (where "sr" means "SCSI ROM") and counting up.

[20]

Compatibility

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Many DVD players do not support any UDF revision other than version 1.02. Discs created with a newer revision may still work in these players if the ISO 9660 bridge format is used. Even if an operating system claims to be able to read UDF 1.50, it still may only support the plain build and not necessarily either the VAT or Spared UDF builds.

Mac OS X 10.4.5 claims to support Revision 1.50 (see man mount_udf), yet it can only mount disks of the plain build properly and provides no virtualization support at all. It cannot mount UDF disks with VAT, as seen with the Sony Mavica issue.[21][22] Releases before 10.4.11 mount disks with Sparing Table but does not read its files correctly. Version 10.4.11 fixes this problem.[23][24]

Similarly, Windows XP Service Pack 2 (SP2) cannot read DVD-RW discs that use the UDF 2.00 sparing tables as a defect management system.[25] This problem occurs if the UDF defect management system creates a sparing table that spans more than one sector on the DVD-RW disc. Windows XP SP2 can recognize that a DVD is using UDF, but Windows Explorer displays the contents of a DVD as an empty folder. A hotfix is available for this[26] and is included in Service Pack 3.[25]

Due to the default UDF versions and options, a UDF partition formatted by Windows cannot be written under macOS. On the other hand, a partition formatted by macOS cannot be directly written by Windows, due to the requirement of a MBR partition table. In addition, Linux only supports writing to UDF 2.01. A script for Linux and macOS called format-udf handles these incompatibilities by using UDF 2.01 and adding a fake MBR;[27] for Windows the best solution is using the command-line tool format /FS:UDF /R:2.01.

Tools for repairing UDF file systems and for modifying their properties have been made for some operating systems. On Windows, the chkdsk tool is able to repair UDF file systems (if not too damaged, like any file system). A similar tool for Linux, udffsck, was under development by Vojtech Vladyka as an extension of fsck, and it was planned to be added to the udftools package. However, it was abandoned by its developer in 2018.[28]

A tool for toggling the write protection flag, udftune, was under development by Johannes Truschnigg in 2023 but could not be merged into the main udftools package because its maintainer Pali Rohár has reportedly lost access to his GitHub account.[19]

Table of operating systems
  • Unless otherwise noted, read and write support means that only the plain UDF build is supported, but not the VAT and spared build.
  • Support for "read" means that a UDF formatted disk can be mounted by the system. It enables the user to read files from the UDF volume using the same interface that is used to access files on other disks connected to the computer.
  • Support for "write" means that, in addition to reading files from a mounted UDF volume, data such as files can be modified, added, or deleted.
UDF revision (read + write) Non-plain
Operating system 1.02 1.50 2.0x 2.50 2.60 VAT Sparing tables Note
AIX 5.2, 5.3, 6.1 Yes Yes No No 1.5 is default[29]
AmigaOS 4.0 Yes Yes
BeOS/magnussoft ZETA/Haiku Yes Yes Yes Yes Yes
OS/2 (including eComStation and ArcaOS) Yes Additional fee drivers on OS/2.
FreeBSD 5.0 and newer read only read only[30] No No No No Yes
Linux kernel 2.2 No No No No No No No
Linux kernel 2.4 Yes Yes Yes[h] No No Yes Yes
Linux kernel 2.6.0–2.6.25 Yes Yes Yes No No Yes Yes Kernel versions prior to 2.6.10 supported fewer media types.
Linux kernel 2.6.26 and newer Yes Yes Yes read only[31] read only[12]: 10  Yes Yes Permission-related mounting options added in 2.6.30.[32] Auto-detection of UDF file system on hard disk is supported since version 2.6.30. Auto-detection of UDF file system on disk images was fixed in 4.11.
Mac OS 8.1–8.5 Yes No No No No No No Some earlier versions of Mac OS, such as 7.5, 7.6, and 8.0 are also supported via third-party utilities, along with additional UDF version support for 8.1 and 8.5.[i]
Mac OS 8.6, Mac OS 9 Yes Yes No No No No No Additional UDF version support via third-party utilities.[i]
Mac OS X 10.0–10.3 Yes Yes[33] No[33] No No No No
Mac OS X 10.4 Yes Yes Yes No[j][34] No[k] No Yes[l] Can create UDF 1.50 (plain build) volumes using the drutil utility.
Mac OS X 10.5 and newer Yes Yes Yes Yes[35][34] read only[35][36] Yes Yes To create, use newfs_udf utility.
NetBSD 4.0 read only[37] read only read only read only read only Yes Yes Reading multi-session VAT, spared and metapartition variants
from all CD, DVD and BD variants as well as HDD and Flash media.
NetBSD 5.0 Yes Yes Yes Yes Yes Yes Yes Write support for all builds and media including multi-session VAT.[38] Create new with newfs_udf.
Limited writing on 2.50/2.60 (due to needing pre-allocated, fixed sized metadata partition).[39]
NetWare 5.1
NetWare 6
OpenBSD 3.8–3.9 read only[40] No No No No No No
OpenBSD 4.0–4.6 read only read only[41] No No No Yes[41] No
OpenBSD 4.7 read only read only read only read only[42] read only[42] Yes Yes
Solaris 7 11/99+ Yes Yes
Solaris 8/9/10 Yes Yes
DOS, FreeDOS, Windows 3.11, Windows 95, Windows 95 OSR2+ and other DOS based OS No[43] No No No No No No No native support. Filesystems that have an ISO9660 backward compatibility structure can be read.
Windows 98, Windows Me read only and only for CD/DVD optical disks[44][45][46][43] No No No No No No Additional read/write support via third party utilities[m]
Windows 2000 read only[46][47][48][49][43] read only No No No No No Additional read/write support via third party utilities[m]
Windows XP/Server 2003 read only[48][49][43] read only read only No No Yes Yes[n] Additional read/write support via third party utilities[m]
Windows Vista Yes[50][51][49][43] Yes Yes Yes read only[50][51][49][43] Yes Yes Referred to by Microsoft as Live File System. Requires fake MBR partition on non-optical devices.
Windows 7 and newer Yes[43] Yes Yes Yes Yes Yes Yes
Operating system 1.02 1.50 2.0x 2.50 2.60 VAT Sparing tables Note
UDF revision (read + write) Non-plain

See also

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Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Universal Disk Format

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from Grokipedia
The Universal Disk Format (UDF) is an open, vendor-neutral designed for storing and interchanging data on optical mass-storage media, such as CDs, DVDs, and Blu-ray discs, providing support for read-only, write-once, and rewritable formats across multiple operating systems including Windows, macOS, and . Developed by the Optical Storage Technology Association (OSTA) from 1992 to 2006, UDF serves as a practical subset of the ISO/IEC 13346 standard (originally ECMA-167, first published in 1995), aiming to simplify implementation while ensuring broad compatibility and minimizing complexity for data exchange. In 2022, OSTA transferred the UDF copyright to , which now maintains the specification through technical reports detailing its revisions. UDF's primary purpose is to enable seamless data interchange and advanced functionality on optical media, succeeding the more limited file system by supporting rewritable discs, large file sizes up to 2 TB, filenames up to 255 characters, and features like file-level security, access control lists, and real-time file handling for applications such as video recording. It also incorporates mechanisms like Virtual Allocation Tables (VAT) for simulating rewritability on write-once media, pseudo-overwrite for efficient updates, and metadata partitions for performance optimization in clustered environments. Key revisions of UDF have progressively enhanced its capabilities: version 1.02 (1996) provided basic support for ; 1.50 (1997) introduced VAT and UDF Bridge with ; 2.00 (1998) added stream files and power calibration; 2.01 (2000) enabled real-time files; 2.50 (2003) included metadata partitions; and 2.60 (2005) supported pseudo-overwrite, with UDF 2.5 specifically adopted for Blu-ray Discs (BD-ROM, BD-R, BD-RE) to ensure between and PCs. Additionally, Secure UDF (2002) incorporates for protected content. In practice, UDF is the official file system for DVD and Blu-ray formats, facilitating applications from video authoring (e.g., and BDAV) to general data storage, and it supports hybrid bridges with for on mixed-media discs.

Introduction

Definition and Standards

The Universal Disk Format (UDF) is an open, vendor-neutral designed for on a broad range of optical media, including read-only, write-once, and rewritable formats such as CDs, DVDs, and Blu-ray discs. It facilitates data interchange across diverse operating systems like Windows, macOS, and by providing a standardized structure for organizing files and volumes. UDF is based on the international standards ISO/IEC 13346 (Volume and File Structure for Write-Once and Rewritable Media using Non-Sequential Recording), published in 1995, and its equivalent ECMA-167, first standardized by in 1992 with revisions in 1994 and 1997. These standards define the core volume and file structures for non-sequential recording media, with UDF implementing a practical subset to ensure interoperability and simplicity. The Optical Storage Technology Association (OSTA) played a central role in developing and maintaining UDF from 1992 to 2006, focusing on maximizing data interchange while minimizing implementation complexity; in 2022, OSTA transferred the UDF copyright to for ongoing integration with ECMA-167. In December 2023, published ECMA TR/112 as a comprehensive technical report that compiles the UDF specifications across its major revisions (from 1.02 to 2.60, plus Secure UDF 1.00) into eight parts, serving as a definitive reference for implementers and ensuring the format's continued relevance for . UDF acts as a successor to the earlier standard, extending support for writable and rewritable media in modern optical applications while maintaining compatibility through formats like UDF Bridge discs.

Key Features and Advantages

The Universal Disk Format (UDF) supports incremental updates, multi-session recording, and packet writing, enabling efficient data management on both write-once and rewritable optical media such as CD-R, DVD-R, and DVD-RW. Incremental updates allow for small changes to files without rewriting the entire disc, facilitated by mechanisms like the Virtual Allocation Table (VAT) on write-once media, which remaps sectors to simulate rewritable behavior. Multi-session recording permits adding data in multiple sessions, extending the usability of partially recorded discs, while packet writing breaks data into small packets for drag-and-drop operations, supporting fixed or variable packet sizes aligned to media boundaries. These capabilities make UDF suitable for dynamic recording environments, including sequential recording modes on media like Blu-ray Disc Recordable (BD-R). Compared to the file system, primarily designed for read-only CD-ROMs, UDF offers significant advantages including support for files up to 16 exabytes (2^64 - 1 bytes, theoretically), filenames up to 255 characters, and character encoding for international interoperability. imposes restrictions such as 8.3 filenames in its basic form or up to 64 characters with extensions like Joliet, along with a 4 limit per file and no native writability, requiring full disc remastering for changes. In contrast, UDF enables direct writability and modifications without remastering, along with rich and security at the file and directory levels, enhancing its utility for modern storage needs. Additionally, UDF incorporates the VAT for write-once media to handle logical overwrites and sparing tables for defect management on rewritable media, where defective sectors are remapped to spare areas to maintain without interrupting access. UDF's vendor-neutral design, developed under the Optical Storage Technology Association (OSTA) and standardized as ISO/IEC 13346, ensures broad across devices, operating systems, and , from computers to DVD players. This openness allows seamless data exchange without proprietary dependencies, supporting formats like named streams and real-time files for multimedia applications. Furthermore, UDF provides backward compatibility with through hybrid or bridge formats, where both file systems coexist on the same disc, enabling legacy readers to access content while leveraging UDF's advanced features on compatible systems.

History and Development

Origins and Standardization

The development of the Universal Disk Format (UDF) was initiated in 1992 by the Optical Storage Technology Association (OSTA), a consortium of optical storage industry leaders, to overcome the limitations of the file system, which was primarily designed for read-only media and proved inadequate for the rewritable and higher-capacity requirements of emerging DVD technology. UDF emerged from collaborative efforts on ISO/IEC 13346, which standardized volume and file structures for non-sequential recording on optical media, with work starting in the early . OSTA released the initial UDF specification in September 1995 as a platform-independent solution for data interchange on optical media, aiming to support both sequential and non-sequential recording methods while ensuring with CD-era formats. Early efforts involved close collaborations among key industry groups, including , , and , which were instrumental in unifying competing proposals and advancing the DVD standard through the newly formed . These partnerships facilitated the integration of UDF into the broader ecosystem of , transitioning from the constraints of CD-based file systems like to more versatile formats suitable for DVD and later Blu-ray compatibility. In 1996, the DVD Forum and related consortia adopted UDF version 1.02 as the mandatory file system for DVD-Video and DVD-Audio formats, ensuring standardized playback and data access across consumer electronics and computer systems. This adoption marked UDF's initial standardization as a practical subset of the ISO/IEC 13346 international standard, which defines volume and file structures for write-once and rewritable media using non-sequential recording. The specification emphasized interoperability for multimedia content, laying the groundwork for subsequent revisions that expanded its scope.

Revisions and Evolution

The Universal Disk Format (UDF) has evolved through a series of revisions managed by the Optical Storage Technology Association (OSTA), with each version building on the ISO/IEC 13346 standard to address advancements in optical media capabilities, such as rewritability, larger capacities, and applications. These updates focused on enhancing flexibility, defect management, and compatibility across read-only, write-once, and rewritable discs. UDF 1.00, released on October 24, 1995, established the foundational structure for DVD media, defining core volume and file descriptors while aligning with the emerging DVD-ROM specifications. UDF 1.02, issued in August 1996, incorporated refinements for compliance, including updates to allocation descriptors and path components to ensure seamless playback and data interchange on commercial video discs. UDF 1.50, published in February 1997, introduced the Virtual Allocation Table (VAT) to enable packet writing on write-once media like , along with sparing tables for defect management on rewritable media and UDF Bridge support for hybrid compatibility. This revision marked a shift toward practical use in consumer recording devices, facilitating incremental writing without full disc reformatting. UDF 2.00, released in April 1998, added support for stream recording files, lists, and power calibration features, aligning with ECMA-167 3rd edition to handle named streams and 2.0 for international character sets. UDF 2.01, issued in March 2000, provided minor fixes and enhancements for , including real-time file support, tag serial numbers for integrity, and improved disaster recovery mechanisms. UDF 2.50, approved in April 2003, extended compatibility to high-capacity formats like Blu-ray Disc and , incorporating metadata partitions for clustering, optional duplication, and real-time recording optimizations to support larger volumes up to 50 GB or more. This version was adopted as the primary for Blu-ray, enabling efficient handling of multimedia streaming and high-definition content. UDF 2.60, released in March 2005, introduced Pseudo OverWrite functionality for write-once media, allowing simulated rewrites on BD-R discs to mimic rewritable behavior without physical overwriting. No major revisions have been issued since 2005, as OSTA ceased active development in 2006; however, in 2022, OSTA transferred UDF copyrights to , which reaffirmed the existing specifications without alterations in ECMA TR/112 (December 2023), ensuring ongoing relevance for modern amid evolving demands for larger capacities and .

Technical Specifications

File System Architecture

The Universal Disk Format (UDF) architecture is defined as a profile of the ISO/IEC 13346 standard (also known as ECMA-167), providing a structured framework for volume recognition, file organization, and data interchange on optical media. At its core, the architecture relies on the Anchor Volume Descriptor Pointer (AVDP), which locates the Volume Descriptor Sequence (VDS) by pointing to its main and reserve extents, typically recorded at logical sector 256, the last addressable sector (N), and N-256 to ensure accessibility even if parts of the volume are damaged. The VDS itself is a sequence of descriptors, including the Primary Volume Descriptor for volume identification and the Logical Volume Descriptor for defining the logical volume structure, with the prevailing descriptors determined by the highest recording sequence number. The File Set Descriptor then anchors the file hierarchy within the logical volume, specifying the location and extent of the file entry, enabling the traversal of files and directories. Metadata organization in UDF centers on partition maps, integrity sequences, and extent allocation to manage the division and consistency of volume data. Partition maps, such as Type 1 for direct physical partitions and Type 2 for indirect or virtual mappings, associate physical partitions with logical volumes, supporting features like defect management through sparable partitions. Integrity sequences, recorded as part of the VDS, maintain volume state through descriptors that track modifications and ensure , with a minimum extent length of 16 logical sectors for main and reserve sequences. Extent allocation employs descriptors—short, long, or extended—to specify the location and length of data blocks in logical sectors, allowing files to be represented as contiguous or fragmented extents up to a maximum length of 230−12^{30} - 1 sectors, rounded to the logical block size. Data allocation methods in UDF utilize space bitmaps to track free and allocated space within partitions, where each bit represents the allocation status of a logical block, excluding the first 32 kilobytes of volume space to avoid interference with boot sectors. Metadata partitions, introduced in UDF Revision 2.50, dedicate specific areas for storing file system metadata in clustered form, including a metadata file for file entries, an optional mirror file for redundancy, and a bitmap file for allocation tracking, identified by the entity tag "*UDF Metadata Partition" to enhance performance on rewritable media. UDF architecture supports non-sequential recording on write-once and rewritable media through mechanisms like the Virtual Allocation Table (VAT) for simulating overwrites on sequential media and Access Type 4 partitions for , enabling flexible data placement without requiring contiguous sectors. It also accommodates multi-session volumes, where multiple recording sessions form a volume set with a shared identifier, and the last session holds the valid UDF structures, with AVDP locations adjusted to session starts (e.g., S + 256) during incremental recording. The Integrity Volume Descriptor, part of the Logical Volume Integrity Sequence, facilitates crash recovery and volume validation by recording the volume's status (open or closed), free space tables, and size tables, ensuring that implementations can verify consistency and recover from interruptions, with a minimum extent of 8 kilobytes on rewritable media. Variant builds of UDF adapt this core architecture for specific media constraints, such as VAT for write-once discs.

Volume and File Limits

The Universal Disk Format (UDF) supports variable logical sector sizes that are multiples of 512 bytes, directly influencing the maximum volume capacity. For a standard 512-byte sector size, the maximum volume size is limited to 2 tebibytes (TiB), calculated from 2³² - 1 logical sectors. With a 2 kibibyte (KiB) sector size, commonly used in optical media such as DVDs, this extends to 8 TiB. Larger 4 KiB sectors, more typical for non-optical storage like hard drives, allow up to 16 TiB per volume. These limits stem from the 32-bit addressing of logical blocks in the specification, enabling scalability beyond traditional optical disc capacities while maintaining compatibility. UDF's design accommodates extremely large individual files, with a maximum file size of 16 exbibytes (EiB), or 2⁶⁴ - 1 bytes, due to 64-bit length fields in file entries. This eliminates the 4 gibibyte (GiB) per-file restriction inherent in older file systems like FAT32, making UDF suitable for storing and other large media files without fragmentation or splitting. Filename lengths in UDF are capped at 255 bytes when encoded in UTF-16, supporting international characters while preventing overflow in descriptor fields; the overall path length is limited to 1023 bytes to ensure efficient traversal across directories. Directories can hold up to entries, balancing performance and metadata overhead in file allocation tables. Sector size choices enhance UDF's versatility across media types: optical discs are typically fixed at 2 KiB sectors for correction compatibility, capping practical volumes at around 8 TiB but optimizing read speeds; in contrast, non-optical media like flash drives or HDDs can employ larger sectors (e.g., 4 KiB) to achieve higher capacities and better for bulk . This adaptability, rooted in the file system's architecture, allows UDF to scale for diverse applications without media-specific redesigns.

Character Encoding

The Universal Disk Format (UDF) employs OSTA Compressed , designated as OSTA CS0, as its primary character set for encoding filenames, metadata, and other textual elements. This encoding scheme represents a compressed variant of , specifically utilizing d-characters from Unicode 2.0 or later versions up to 4.0, where each character is stored in either 8-bit or 16-bit units preceded by a compression identifier (8 for 8-bit or 16 for 16-bit). The format ensures efficient storage while maintaining compatibility with international text requirements, and it is mandated for use in key structures such as file identifiers and volume descriptors. OSTA CS0 explicitly excludes byte order marks, such as U+FEFF and U+FFFE, to prevent encoding ambiguities during interchange. For the 16-bit compression mode, characters are encoded in big-endian byte order, effectively aligning with UTF-16 Big Endian (UTF-16 BE) for multi-byte sequences, which supports a broad range of scripts beyond basic Latin characters. This reliance on UTF-16 BE in higher compression modes facilitates handling of multi-byte characters, allowing UDF to represent complex international text without loss of fidelity in compliant implementations. To ensure cross-platform consistency, UDF implementations are recommended to apply Unicode Normalization Form C (NFC) to strings before encoding, as outlined in Unicode Standard Annex #15, which canonicalizes character compositions and decompositions for interoperability. This normalization process helps mitigate variations in how multi-byte characters are represented across different systems, promoting reliable display and processing of filenames containing diacritics or combined glyphs. Additionally, OSTA CS0 maintains with ASCII and ISO/IEC 646 standards through 's inclusion of these as subsets, enabling legacy systems to interpret basic English text without additional translation. Despite its strengths, OSTA CS0 in earlier UDF revisions exhibits limitations, particularly the absence of full support for Unicode versions 11.0 and beyond, restricting access to newer characters and scripts introduced post- 4.0. This constraint arises from the encoding's foundational tie to older Unicode iterations, potentially requiring extensions or updates in modern deployments to accommodate evolving needs, such as expanded or supplementary planes. Filename lengths, when incorporating these multi-byte encodings, are measured in bytes rather than characters, which can affect effective limits in diverse linguistic contexts.

Implementation Variants

Plain Build

The Plain Build represents the fundamental implementation of the Universal Disk Format (UDF), designed for media that support direct overwrites or random access, such as pressed (ROM) discs like DVD-ROM and hybrids. This build adheres closely to the core ISO/IEC 13346 standard, enabling efficient data storage and retrieval without specialized mechanisms for write-once or defect-prone media. Key components of the Plain Build include direct extent allocation, where files and directories are mapped to contiguous or non-contiguous blocks on the disc using allocation descriptors, allowing straightforward addressing of locations. Free space management relies on space bitmaps within the partition descriptor, which track allocated and available sectors in bit form for quick querying and allocation during formatting or initial writing. Unlike variants for sequential media, it omits the Virtual Allocation Table (VAT) and sparing tables, resulting in a streamlined structure without virtual mappings or pseudo-overwrite simulations. This build is particularly suited for use cases involving read-only or pre-mastered optical media, such as DVD-ROM for software distribution or hybrids combining compatibility with extended UDF features, where incremental writes are not required. The metadata structure consists of the file set descriptor and partition maps within the main volume descriptor sequence, without additional layering for updates. The advantages of the Plain Build lie in its simplicity, which facilitates rapid read access and minimal processing overhead compared to more complex builds, making it ideal for high-performance playback scenarios. It was introduced in early UDF revisions, such as version 1.00 in , to provide a baseline for cross-platform interchange on rewritable and ROM media. In contrast to the VAT Build designed for write-once media, the Plain Build prioritizes direct access efficiency for fixed-content discs.

VAT Build

The VAT Build, introduced in Universal Disk Format (UDF) version 1.50 in February 1997, enables incremental writing on write-once media such as DVD-R and by simulating updates through appended data rather than physical overwrites. This mechanism supports packet-writing applications, allowing drag-and-drop file operations on sequentially recordable discs without requiring full rewrites of existing structures. The Virtual Allocation Table (VAT) is structured as a dedicated file described by a File Entry Initial Content Block (ICB) with File Type 248, typically positioned at the end of the disc or session. It consists of a header containing the Logical Volume Identifier and the number of virtual sectors (files and directories), followed by an of 32-bit unsigned entries that each virtual block to its corresponding physical or logical block on the media. Unused virtual sectors are marked with the value 0xFFFFFFFF, and the table is updated and recorded as the final element in each writing session to ensure consistency. In the incremental writing process, modifications to files or directories are handled by appending new data extents to available space on the disc, while the VAT is revised to redirect logical references to these new locations, preserving all prior data intact. For small updates, the entire VAT may be embedded within the ICB; larger changes require multiple sectors allocated via long allocation descriptors. This approach, combined with a Virtual Partition Map, allows the to maintain a consistent view of the volume across multiple sessions, enhancing usability for applications like or content authoring on write-once optical media. A key limitation of the VAT Build is the progressive growth of the table's size with each update or new session, as it must encompass all virtual addresses for the volume's files and directories, potentially leading to increased disc space consumption and read overhead over time. Additionally, the requirement that the VAT ICB serve as the last recorded sector in any transaction can complicate error recovery if interruptions occur before completion.

Spared Build

The Spared Build, also referred to as the RW build, is a configuration of the Universal Disk Format tailored for rewritable optical media such as DVD-RW, , and Blu-ray RW, where it incorporates sparing tables to handle defective sectors and maintain across multiple write-erase cycles. This build employs a Sparable Partition Map of type 2 to define the partition's sparing parameters, including packet length and locations of sparing tables, enabling the to map logical blocks affected by defects to alternative physical locations. Unlike plain builds, it addresses the dynamic nature of rewritable media by providing host- or drive-managed defect handling, ensuring an apparent defect-free presentation to applications. The sparing mechanism utilizes certified sparing areas, which are pre-allocated during disc formatting, and alternate sparing areas, which are dynamically as needed from unallocated space. Sparing tables, maintained with 1 to 4 redundant copies stored in physically separated locations for , record pairs of original and mapped sector addresses, sorted by original location and protected by sequence numbers to track updates. For DVD-RW, this is primarily host-managed, with the operating system updating tables during writes; in contrast, and Blu-ray RW rely on drive-managed linear replacement algorithms that allocate default spare areas at formatting and expand them if exhausted, minimizing host intervention. Introduced in UDF revision 1.5 to accommodate rewritable media characteristics, this mechanism supports packet sizes like 32 KB (16 sectors) for DVD-RW, aligning relocations to packet boundaries. In UDF 2.50 and later, the Spared Build was enhanced with a dedicated metadata partition for efficient access to structures, including the File Set Descriptor, Integrity Sequence Descriptors, and directory entries. This partition includes a Metadata File for primary storage, an optional Mirror File that duplicates contents when the Duplicate Metadata Flag is set (enhancing against localized damage), and a Metadata Bitmap File to track free blocks within the partition. Such mirroring and facilitate rapid repairs and maintain consistency during repeated rewrites, with unallocated space bitmaps further indicating available space sets for DVD-RW and . Defect detection and relocation occur dynamically during write operations: the host or drive identifies errors via read-after-write verification or formatting checks, marking defective packets in the Non-Allocatable Space Stream (for DVD-RW) with addresses like 0xFFFFFFF0, then relocating data to a spare area and updating the sparing table with the new mapped location. This process ensures seamless data recovery without interrupting access, with redundant table copies preventing total loss from isolated failures. The advantages of the Spared Build lie in its robustness for media subjected to frequent overwrites, enabling reliable long-term use in applications like video recording on and high-density data storage on Blu-ray RW, while promoting cross-platform data interchange through standardized defect management. In UDF 2.60 (published around 2005), the Pseudo OverWrite (POW) feature was introduced to simulate overwrite operations on compatible media by redirecting writes to the next writable address or spare areas tracked via sparing tables, though it is primarily applied to write-once formats; this complements the native overwrite capability of rewritable media in spared configurations.

Applications and Compatibility

Primary Uses in Optical Media

The Universal Disk Format (UDF) serves as the primary for various optical media, enabling efficient storage and retrieval of data, video, and multimedia content across consumer and professional applications. In DVD formats, UDF is integral to discs, which employ a UDF Bridge format combining UDF version 1.02 with to ensure compatibility with both legacy drives and modern DVD players. This hybrid approach supports the organization of video streams, audio, and navigation files on read-only DVD-ROM media, while also facilitating authoring on recordable DVD-R and rewritable DVD-RW discs for data storage and . For Blu-ray Discs, UDF version 2.50 is mandatory, providing robust support for high-capacity storage in BD-ROM (read-only), BD-R (recordable), and BD-RE (rewritable) formats. This version accommodates advanced features like metadata partitioning and stream recording, essential for and interactive content, including 4K applications that extend to 100 GB or more per disc. The specification ensures seamless interchange between players and personal computers, with optimized for Blu-ray's 25 GB and 50 GB layer capacities. UDF also enables packet writing on CD-R and CD-RW media, allowing incremental file addition, deletion, and modification in a drag-and-drop manner similar to removable storage devices. Version 1.50 introduces mechanisms like the Virtual Allocation Table (VAT) to manage sequential recording on these lower-capacity discs, making them suitable for everyday data backup and portable file storage despite their limited endurance compared to DVD or Blu-ray. The now-defunct format utilized UDF version 2.50 for its high-density read-only (HD DVD-ROM) and rewritable variants, supporting up to 30 GB per layer for advanced video titles before the format's discontinuation in 2008 amid competition with Blu-ray. In niche and emerging applications, UDF appears in archival optical media for long-term data preservation and hybrid discs combining optical layers with other storage types, though adoption remains limited outside traditional optical domains. Additionally, UDF sees sporadic non-optical use in and hard disk drives for cross-platform compatibility in specialized removable storage scenarios.

Support in Operating Systems

Universal Disk Format (UDF) enjoys varying levels of native support across major operating systems, primarily driven by its role in optical media compatibility, though adoption extends to some non-optical uses like flash drives in select implementations. Support typically includes read access for basic UDF revisions, with write capabilities depending on the OS kernel and hardware drivers, often limited to specific UDF versions and build types such as plain UDF without advanced features like Virtual Allocation Tables (VAT).

Windows

Microsoft Windows provides native UDF support starting with , where read access is available for UDF versions 1.50 and higher on optical media. Beginning with and continuing through , full read and write support extends to UDF 2.60, though write operations are generally restricted to UDF 2.50 due to driver limitations in handling higher revisions on non-optical volumes. This support is integrated via the kernel's drivers, enabling seamless mounting of UDF-formatted DVDs and Blu-ray discs without additional software. However, compatibility can vary with USB-attached UDF volumes, where post-2020 updates have occasionally required driver tweaks for reliable mounting.

Linux

The Linux kernel includes UDF support through the udf module, introduced in version 2.4 and matured in subsequent releases, allowing read access to UDF volumes up to version 2.60. Write support is available for earlier versions such as 2.01 using tools like udftools for formatting and management, but full write functionality for UDF 2.50 and 2.60 remains limited or experimental as of kernel 6.12 in 2025, particularly for advanced features like metadata partitions on media requiring random sector writes, such as DVD+RW drives. Automatic mounting of UDF filesystems has been reliable since kernel 2.6.30, provided the block size aligns with the device's logical sector size.

macOS

macOS offers read and write support for UDF starting from OS X 10.5 (), covering up to UDF 2.50 via built-in kernel extensions, suitable for optical media like DVDs. For UDF 2.60, support is partial, with read access generally functional but write operations requiring command-line tools like newfs_udf and limited to specific hardware configurations. Apple's app integrates UDF handling for disc images and burns, though advanced features may necessitate third-party drivers for professional workflows, such as importing from UDF-formatted cards.

Other Operating Systems

BSD variants, including and , provide UDF read support through dedicated filesystem drivers, with 's implementation handling basic UDF on optical media since early 2000s releases, though write access remains experimental and limited to plain builds. Embedded systems like PlayStation OS, based on a FreeBSD kernel, support UDF 2.50 for Blu-ray and DVD playback, enabling read access to game and media discs without native write capabilities. Android offers limited read-only support for UDF, primarily via kernel modules in custom ROMs for optical or USB media, but lacks native write functionality in stock versions up to Android 15. Common issues in UDF support across operating systems include incomplete handling of VAT in older kernels, which prevents efficient updates on write-once media like , and limited Packet Writing (POW) compatibility that can cause mount failures on sequential media. Additionally, universal 4K sector handling was not standardized until the 2020s, with pre-2020 kernels in and Windows often requiring emulation modes that reduce performance on drives formatted as UDF.
Operating SystemRead SupportWrite SupportMax UDF VersionKey Limitations
UDF 1.50+UDF 2.502.60Write limited to 2.50; USB mount issues post-updates
(2.4+)UDF 2.60UDF 2.01 (full), partial for 2.50+2.60 (read)Write limited to 2.01 reliably; experimental for higher versions; needs random-write hardware and tools
macOS (10.5+)UDF 2.50UDF 2.502.60 (partial)Command-line for 2.60; hardware-dependent
/Basic UDFExperimental2.01No full write; optical-focused
PlayStation OSUDF 2.50None2.50Read-only for media discs
AndroidBasic readNone2.01Custom ROMs only; no stock write

Tools and Software

Creation and Management Tools

The udftools package provides essential command-line utilities for users to create and manage UDF filesystems, with mkudffs serving as the primary tool for formatting devices or generating UDF images. It supports revisions up to 2.01 for non-write-once media and higher revisions like 2.50 for write-once scenarios with appropriate metadata partitions, while enabling plain, VAT (Virtual Allocation Table), and spared builds through options such as -V for VAT inclusion and -t for build type specification. Users can specify the UDF revision via the -r flag (e.g., 0x0201 for 2.01), select media types like write-once or pseudo-overwrite, and configure block sizes or volume identifiers to tailor the filesystem structure. On Windows, offers robust support for burning UDF-formatted optical discs, allowing users to create data discs with selectable UDF revisions such as 1.02 for broad compatibility or 2.50/2.60 for advanced features like Blu-ray support. The tool's options tab enables customization of the to UDF-only or hybrid modes, with built-in verification to ensure burn integrity, making it suitable for authoring large archives on DVD or Blu-ray media. Similarly, facilitates UDF disc creation through its data compilation interface, where the UDF tab allows selection of packet sizes, bridge formats with , and revisions up to 2.01 or higher for specific media types. Windows also includes native formatting for DVD-RW media via the built-in optical drive tools in , which defaults to UDF 1.50 for packet writing compatibility. For macOS, the newfs_udf command-line utility allows formatting block devices or creating UDF volumes with support for revisions up to 2.60, such as for Blu-ray discs, using options like -r for revision and -v for volume name (e.g., newfs_udf -r 2.60 /dev/diskX). Additionally, hdiutil can manage UDF images and burn to optical media, supporting hybrid formats. For cross-platform needs, growisofs from the cdrkit suite enables multi-session UDF writing to optical drives, particularly useful for incremental backups on DVD+RW or BD-RE media. It integrates with mkisofs or genisoimage to generate hybrid UDF/ sessions, supporting first-session recording of pre-mastered images and subsequent appends via options like -M for medium scanning and -Z for multi-session continuation. Additional tools include PacketCD, a legacy Linux utility for UDF-based packet writing on CD-RW media, which formats discs for drag-and-drop file operations akin to removable storage. For GUI-based authoring on Linux, K3b provides an intuitive interface to create UDF volumes, with project properties allowing selection of UDF as the primary filesystem, revision levels, and options for multi-session or packet modes during burn setup. These tools collectively allow precise control over UDF parameters, such as revision selection for and build type choices (e.g., VAT for write-once emulation), ensuring compatibility across diverse optical media. As of November 2025, no major new open-source UDF tools have emerged beyond updates to existing packages like udftools (last release 2021).

Diagnostic and Repair Tools

Several open-source tools are available for diagnosing and repairing Universal Disk Format (UDF) file systems, particularly within Linux environments where support for optical media is prominent. The udftools package, maintained on GitHub, provides essential utilities for inspecting UDF volumes on block devices or disk images. A key diagnostic tool in udftools is udfinfo, which extracts and displays detailed information about a UDF file system, including volume descriptors, partition maps, and metadata such as labels, UUIDs, and free space availability. This output is formatted for easy parsing and aids in verifying the structural integrity of volumes, such as confirming the presence and validity of Anchor Volume Descriptor Pointers (AVDPs) that locate essential descriptors on the disc. For testing purposes, wrudf offers an interactive shell to perform operations like copying, removing, or listing files on an existing UDF file system, allowing users to write test files and assess read-write functionality without altering production data. Repair capabilities remain limited in open-source tools. The udffsck utility, intended as a file system check and repair tool analogous to for other formats, has been in development since at least but remains incomplete and stalled, functioning primarily as a placeholder in distributions like Gentoo without full detection or fixing features as of November 2025. Similarly, udftune, proposed for adjusting parameters such as sparing thresholds in UDF volumes, is under development but not yet implemented in the udftools package as of November 2025. These gaps highlight ongoing challenges in maintaining UDF repair tools for , where users often resort to manual interventions or third-party software for recovery. For cross-platform diagnostics and , IsoBuster serves as a commercial utility that scans UDF-formatted optical media, including CDs, DVDs, and Blu-ray discs, to extract files even from partially damaged or incomplete sessions. It specializes in reconstructing lost structures, such as Virtual Allocation Tables (VATs) used in sequential-write variants, and handles partial burns by identifying valid sessions and recovering file names, timestamps, and contents from fragmented . Common diagnostic tasks with these tools involve validating VAT integrity to ensure proper file mapping in write-once media, locating AVDPs to access core descriptors, and confirming session boundaries to mitigate issues from interrupted multi-session recordings. Such checks are crucial for VAT builds, where incomplete updates can render volumes unmountable.

References

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  2. https://learn.microsoft.com/en-us/windows/win32/imapi/disc-formats
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  29. https://github.com/pali/udftools/issues/43
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  32. https://manpages.ubuntu.com/manpages/jammy/man8/mkudffs.8.html
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  36. https://linux.die.net/man/1/growisofs
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  38. https://wiki.ubuntu.com/PacketCD
  39. https://help.ubuntu.com/community/K3b
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  42. https://man.archlinux.org/man/udfinfo.1.en
  43. https://man7.org/linux/man-pages/man1/wrudf.1.html
  44. https://bugs.gentoo.org/558616
  45. https://github.com/pali/udftools/issues/21
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  47. https://www.isobuster.com/help/recovery
  48. https://www.isobuster.com/help/udf_file_system_settings_scan_options
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