More properly known as ST506/412, this was the first hard disk interface standard to be used in microcomputers. It was created by Seagate Technologies in 1980. At that time, the cost for a five megabyte drive was over one thousand dollars.

The hard drive itself was a full height 5 1/4 inch drive, powered by a stepper motor. It connected to the drive controller, a separate plug-in card, through two ribbon cables, a 34-pin control cable and a 20-pin data cable. Although the control cable could be shared between two drives, each drive required its own data cable. Data transmission was serial, at up to 7.5 Mbps(megabits per second). Compared to the up to 100 MBps(megabytes per second) transfer rate of UDMA5 and the up to 320 MBps transfer rate of SCSI-3, this seems insignificant, but considering the initial drive capacity, the entire contents of the drive could be transfered in less than six seconds.

Two recording methods were used, MFM and RLL. By using RLL, you could get a 50% increase in storage capacity at the cost of some error checking, but use of RLL on non-RLL compatible drives resulted in bad publicity for the standard.

By the late eighties, ST506 had started to be crowded out by ESDI, and by the early nineties, both were made obsolete by SCSI and IDE.