This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
Sony now has 650 MB/side 3.5" MO with direct-overwrite capability (a major step forward in MO). See http://www.sony.co.jp/TechnoGarage/HS/index.html. It's ZCAV, and nominally 1-2 MB/s. There's a reasonable discussion of MO physics and technology here, too. (email@example.com (Mark Holzbach), 10/95)