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magneto-optic disk

An earlier rewritable optical disk that also used magnetic technology. Magneto-optic (MO) disks are housed in removable cartridges that have a 30-year shelf life. Capable of up to a million rewrites, access times are in the sub-25ms range and faster than optical-only CD and DVD media.

Form Factors and Capacities
MO discs came in 3.5" and 5.25" cartridges from 128MB to 9.1GB. Although the 5.25" media were double sided, they had to be removed and flipped over to access the other side. As of 2018, MO products are no longer made, but media and refurbished drives are available (for more information, visit

Laser and Magnetic Writing
A laser heats the optical bit to the Curie point, at which temperature the molecules can be realigned when subjected to a magnetic field. Then, a magnet changes the bit's polarity. The laser is focused on one side of the platter and the magnet on the other, which is why double-sided media must be flipped over.

Laser Reading
Reading is accomplished with a lower-power laser that reflects light from the bits. Depending on the polarity, the light is rotated differently, and that difference is sensed. Writing takes two passes (existing bits are first set to zero, and the data are then written). A single-pass Direct OverWrite method (LIMDOW) was later added, and many drives supported the more costly LIMDOW media. See UDO, Kerr effect and optical disc.

MO Cartridges
The 5.25" cartridges are double sided and have to be flipped to access the other side. The 3.5" cartridges are single sided.

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