The ST-506 and ST-412 (sometimes written ST506 and ST412) were early hard disk drives introduced by Seagate in 1980 and 1981 respectively, that later became construed as hard disk drive interfaces: the ST-506 disk interface and the ST-412 disk interface. Introduced in 1980, the ST-506 was the first 5.25 inch HDD. Its successor, the ST-412, was introduced in 1981 and implemented a refinement to the seek speed, and increased the drive capacity from 5 MB to 10 MB, but was otherwise highly similar.

Beginning with its selection as the hard drive subsystem for the original IBM XT, disk drive controllers supporting the ST-412 interface grew to become ubiquitous in the personal computer industry. The ST-412 interface and its variants were the de facto industry standard for personal computer hard disks until the advent and wider adoption of the IDE or ATA interface in the early 1990s.

The ST-506 HDD was the first 5.25 inch hard disk drive, introduced in 1980 by Shugart Technology (now Seagate Technology). It stored up to 5 megabytes after formatting (153 cylinders, 4 heads, 32 sectors/track, 256 bytes/sector) and cost US$1,500 (equivalent to $5,861 in 2025). The similar, 10-megabyte ST-412 HDD was introduced in late 1981 (with 306 cylinders). The ST225 was introduced shortly thereafter with 20 megabytes and half the height. All three used MFM encoding, a widely used coding scheme. A subsequent extension of the ST-412 interface, the ST-412HP, used RLL encoding for a 50% increase in capacity and bit rate.

The ST-506 drive connected to a computer system through a disk controller. The ST-506 interface between the controller and drive was derived from the Shugart Associates SA1000 interface, which was in turn based upon the floppy disk drive interface, thereby making disk controller design relatively easy.

The ST-412 interface was adopted by numerous HDD manufacturers such that the interface became a de facto industry standard for disk drives well into the 1990s.

The limitations of the ST-412 interface are 5 million transitions per second maximum on data lines, 16 heads, 4 drive units and a 20-foot (6.1 m) cable length. The standard channel code for the ST-412 (and ST-506) is MFM with one data bit per transition for a data rate of 5 Mbit/s. The ST-412HP RLL variant averages 1.5 data bits per transition for a data rate of 7.5 Mbit/s.

In the ST-506 interface, the drive connects to a controller card with two ribbon cables carrying signals, while a third cable provides power. The two signal cables are the wide 34-pin control cable and the narrow 20-pin data cable. The control cable interface is very similar to the standard Shugart floppy disk interface; like that floppy disk interface, it can support four drives. The data cable carries two (Read, Write) separate unidirectional RS-422 compatible differential signal pairs with 100ohm terminating resistors. As with floppy disk drives, a flux reversal corresponds to a rising edge (the positive line of the differential pair attaining a more positive voltage than the negative pair) of a short pulse that is ended with a falling edge after a fixed timeout and thus a return-to-zero code. Even though the timing of these pulses and flux reversals is completely in the hands of the controller, the system design was for MFM, and user data bits were represented by the absence or the presence of these pulses at specific points in time while the controller was precisely synchronized to the flux reversal stream on the platter. While up to four drives can share a control cable, each drive has its own dedicated data cable connecting it to the hard disk controller (HDC). Most HDCs supported only two drives.

The control card translates requests for a particular track and sector from the host system into a sequence of head positioning commands, including setting the direction of head movement, in or out, and sending individual "STEP" commands to move. Four of the control cable pins, "HD SLCT 0" through "HD SLCT 3", allow the selection among up to 16 heads, although only four are available on the two-platter ST-506. The original ST-506/ST-412 interface defined only two HD SLCT lines, providing supporting for only four heads, but a third HD SLCT line was shortly added to the design; a fourth was added a not much later by redefining the reduced Write Current signal, needed only by very early drives, as HD SLCT 3. Once the heads are properly positioned and the appropriate head is selected, data is read or written serially through a set of pins in the data cable. The limited bandwidth of the data cable was not an issue at the time and is not the factor that limited the performance of the system. However, the unshielded cable can sometimes be susceptible to high levels of noise.