A Magneto-Optical Disc (MO) is a removable, rewritable storage medium, which comes in both 5.25-inch and 3.5-inch sizes. The 3.5-inch size is the more commonly used size with personal computers. The discs come in six different sizes of storage capacity, 128MB, 230MB, 540MB, 640MB, 1.3GB, and 2.3GB. All of these sizes, with the single exception of 2.3GB, conform to ISO standards. The discs, which are contained within a 6mm-thick hard casing that is identical to that used for floppy discs, are used with special drives. The discs consist of a magnetic recording layer placed between a reflective layer and a protective layer made of thin film aluminum alloy atop a polycarbonate substrate. Digital information is recorded in tracks that form a spiral on the disc.

Data is recorded by heating a given region on the magnetic recording layer using a high-output laser to above the Curie point (200°-300°C), the point at which the body’s magnetic coercivity will decrease. The simultaneous application of a magnetic field will determine the polarization; when the point cools, that polarization is fixed. The reading of information is performed by observing the Kerr effect (a phenomenon that the plane of polarization of the reflected light rotates depending on the polarization) through the projection of laser light that is weak enough to not raise the temperature on to the magnetic recording layer. In comparison to digital recording media in which the polarization is determined at room temperature (such as floppy discs), MO, which requires high heat for polarization to change, is more resistant to magnetic disturbances and ultraviolet light; in accelerated degeneration tests, the projected lifespan of the discs far exceeded that of other recordable media. Also, the drives can be used without sacrificing access speed in performing random access, because they are compatible with the same sort of defect management and OS-level defragmentation processes as used for magnetic recordable media.

In a Windows environment, writing software is unnecessary to recognize the drives as removable media, which instead can be used just as easily as floppy discs and at the same speeds as a hard drive. Because there are now media that come with the media ID (please see the section on DVD-R and DVD-RW) pre-written, in Windows they are compatible with such functions as content encoding using specialized drives and device drivers (which must be installed). Please note that once media has been initialized for either a Mac or Windows environment, it is no longer compatible with the other environment.