The Free On-line Dictionary of Computing (30 December 2018):

magneto-optical disk
magneto-optical drive
M O drive

    (MO) A plastic or glass disk coated with a
   compound (often TbFeCo) with special optical, magnetic and
   thermal properties.  The disk is read by bouncing a
   low-intensity laser off the disk.  Originally the laser was
   infrared, but frequencies up to blue may be possible giving
   higher storage density.  The polarisation of the reflected
   light depends on the polarity of the stored magnetic field.

   To write, a higher intensity laser heats the coating up to its
   Curie point, allowing its magnetisation to be altered in a way
   that is retained when it has cooled.

   Although optical, they appear as hard drives to the operating
   system and do not require a special filesystem (they can be
   formatted as FAT, HPFS, NTFS, etc.).

   The initial 5.25" MO drives, introduced at the end of the
   1980s, were the size of a full-height 5.25" hard drive (like
   in IBM PC XT) and the disks looked like a CD-ROM enclosed
   in an old-style cartridge

   In 2006, a 3.5" drive has the size of 1.44 megabyte
   diskette drive with disks about the size of a regular 1.44MB
   floppy disc but twice the thickness.

   Storage FAQ

(http://cis.ohio-state.edu/hypertext/faq/usenet/arch-storage/part1/faq.html).

   (2006-07-25)