Like CDs, magneto-optical (MO) discs are designed to be read optically with a laser. But unlike CDs, which users can't write to at all, and CD-Rs, which users can write to only once, most MO discs permit the user to write and erase data on a disc multiple times. They accomplish this by means of a special magnetic process that works in conjunction with a laser to store data. The first widely known magneto-optical drive shipped with the NeXT Computer
(1988, lower right) and used 256MB rewritable media. The best-known MO format is the Sony MiniDisc (top middle, 1992), an audio medium that also has a less popular computer-capable cousin known as MD-DATA (upper left). Various MO drives and discs remain in production, but their relatively low capacity and relatively high cost make them niche products.