MO Disk drives use a material that crystalizes in differing orientations depending on the polarity of the local magnetic field. They are written by heating the material above its "Curie Point" (the temperature where magnetic orientation is lost) with a laser. They are read by detecting the polarity of light reflected from a weaker laser. This is somewhat similar to CD-R except that CD-R depends on the state (amorphous or crystalline) rather than the magnetic polarity of the recording material.
Because the magnetic recording area is limited to the area heated by the recording laser, heads do not need to "fly" close to the disk to achieve high recording density. MO media can be removable and head crashes are unlikely. Recording densities are comparable to those of magnetic storage.
MO drives are available in 12 inch, 5.25 and 3.5 inch formats. The first products were introduced in the early 1990s with capacities of a few hundred Megabytes. Drives of 128, 240 and 650MB are available in 3.5 inches. They are reported to be in common use in Japan, less so in Europe and the US. Drives with storage of up to 10GB are available in the 5.25 inch form factor.
Some MO disks come preformatted to odd sector sizes -- e.g. 2048 bytes -- which cause trouble with common PC operating systems.
MO drives are rewritable in principle. I have found no data on limitations on rewriting. Speed is said to be slower than a hard drive. I was unable to find good specs, but partial data for a 1999 5.25 inch drive indicated rotation speeds of 3000-4000 rpm and average seek time around 20ms. Writing is a two step process with 0 bits written on one pass and 1 bits written on the second.
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