An MO drive is a type of disk drive that uses magneto optical (MO) technology to provide high-capacity portable media for computer systems. The drive itself uses two separate processes for reading and writing to a MO diskette, a laser and a magnetic head. Where the laser is used for both reading and writing to the media, the magnetic head is only for writing. As a result, the special media used with a MO drive is capable of extremely long life and very reliable, high-capacity data storage.
The disks for an MO drive strongly resemble typical floppy disks, but are about twice as thick. Inside the diskette's enclosure, the disk platter is composed of a thin layer of ferromagnetic material that is fixed inside a plastic coating. This material acts very similarly to a platter inside a hard disk drive in that there are tracks with tiny sectors magnetized in either a positive or negative state to represent either a one or zero for the digital data stored on the disk. The disks can be formatted using any number of disk file systems, and when inserted into the MO drive, they are seen by a computer's operating system as just another disk drive.
Inside the MO drive, a small sliding door on the diskette is opened and the platter is situated between the drive's laser on the bottom, and the magnetic head hovering overhead. As the platter spins to read the diskette, the laser is set at a low power and the beam bounces off the sectors on the disk depending on their magnetic state. This is a process known as the Kerr effect. When the laser beam hits a positive magnetic state, or a "one," the beam bounces back in one direction, while when hitting a negative magnetic state, a "zero," it bounces back in the opposite direction. The drive then ascertains the content of the disk by watching for the laser's return.
When an MO drive wants to write to a disk, it needs to increase the power of the laser to heat up the ferromagnetic material of the platter. The laser heats the material to what's called its Curie point, the temperature at which it can be affected by magnetic fields. Once a sector has been heated, it can then be written to by the magnetic head on the opposite side of the diskette, which gives a heated sector a positive or negative state. The diskette platter must make two passes inside the drive for the writing process to complete, making the time it takes to write data with an MO drive twice as long as simply reading it. Of course, this process is infinitely repeatable on an MO drive and diskette, making them endlessly rewritable without having to reformat the diskette each time as with traditional floppy drive media.
In many Western countries, the use of MO drive technology has dwindled somewhat since around the year 2000, though they still find use in specific areas. Due to the durability and large capacity of the disks, MO drives are also used for many data archival tasks, where a number of MO disks can be situated inside an enclosure with one or two drives requesting the disks via robotic arms. A server running special software controls requests to the data library and organizes the data across the multiple disks.