3½ Inch Format
Sony introduced their own small-format 90.0 mm × 94.0 mm disk, similar to the others but somewhat simpler in construction than the AmDisk 3-inch floppy. The first computer to use this format was Sony's SMC 70 of 1982. Other than Hewlett-Packard's HP-150 of 1983 and Sony's MSX computers that year, this format suffered from a similar fate as the other new formats; the 5¼-inch format simply had too much market share.
Things changed dramatically when in 1982 the Microfloppy Industry Committee, a consortium ultimately of 23 media companies, agreed upon a 3½-inch media specification based upon but differing from the original Sony design. The first drives compatible with this new media specification were shipped in early 1983. In 1984 Apple Computer selected the format for their new Macintosh computers.Then, in 1985 Atari adopted it for their new ST line, and Commodore for their new Amiga. By 1988 the 3½-inch was outselling the 5¼-inch.
The term "3½-inch" or "3.5 inch" disk and was rounded from the actual metric size of 90 mm.
The 3½-inch disks had, by way of their rigid case's slide-in-place metal cover, the significant advantage of being much better protected against unintended physical contact with the disk surface than 5¼-inch disks when the disk was handled outside the disk drive. When the disk was inserted, a part inside the drive moved the metal cover aside, giving the drive's read/write heads the necessary access to the magnetic recording surfaces. Adding the slide mechanism resulted in a slight departure from the previous square outline. The irregular, rectangular shape had the additional merit that it made it impossible to insert the disk sideways by mistake as had indeed been possible with earlier formats.
The shutter mechanism was not without its problems, however. On old or roughly treated disks the shutter could bend away from the disk. This made it vulnerable to being ripped off completely (which does not damage the disk itself but does leave it much more vulnerable to dust), or worse, catching inside a drive and possibly either getting stuck inside or damaging the drive.