This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
Sony introduced a 2.5" removable cartridge disk drive in 1993 that will hold 140MB. The rewritable version is magneto-optical; there is also a read-only CD-like version. Also manufactured by others, including Sharp, who says it will have a 700 MB version in 1997. Great info available at http://www.hip.atr.co.jp/~eaw/minidisc.html or http://www.io.com/~jamshid/minidisc.html (rdv, 96/7/7)