The ST506 HDD was the first 5.25 inch hard disk drive, introduced in 1980^[3] by Shugart Technology (now Seagate Technology). It stored up to 5 megabytes after formatting (153 cylinders, 4 heads, 26 sectors/track, 256 bytes/sector) and cost US$1,500 (equivalent to $5,328 in 2022).^[4] The similar, 10-megabyte ST412 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 ST412 interface, the ST412HP interface, used RLL encoding for a 50% increase in capacity and bit rate.

The ST506 drive connected to a computer system through a disk controller. The ST506 interface between the controller and drive was derived from the Shugart Associates SA1000 interface,^[5] which was in turn based upon the floppy disk drive interface,^[6] thereby making disk controller design relatively easy.^[3]