The 5.25-inch floppy disk (or ‘minifloppy’) was a magnetic disk format introduced by Shugart Associates in 1976, as a replacement for the 8-inch floppy disk that was considered too large for newer desktop machines.
Like the 8-inch floppy, the 5.25-inch disk consisted of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removed dust particles. Write protection was carried out by affixing an adhesive tab.
By 1978 a number of manufacturers were producing disks, in competing formats including hard and soft-sector versions, and different encoding schemes.
Initially, 5.25-inch disks had a capacity of 110 KB, with a double-density disk introduced in 1978 with a capacity of 360 KB, and quad-density introduced in the early 1980s with a capacity of 720 KB.
In 1984, IBM introduced the high-density 1.2 MB disk in its PC AT, but by the time of the launch of the PS/2 line in 1987 moved to 3.5-inch floppy disks, as Apple had already done with its Macintosh line in 1984. By 1988 the 3.5-inch was outselling it, and by the mid-1990s the 5.25-inch disk had virtually disappeared.
Single sided disks were coated on both sides, but only double sided disks were certified error-free on both sides of the media. Double-sided disks could be used in single-sided drives, one side at a time, by making minor modifications to the disk (such as making another write-protect notch) or buying ready made ‘flippy disks’. More expensive dual-head drives which could read both sides of the disk without turning it over were later produced, and eventually became used universally.