An adaptor cartridge for holding a 90 mm optical disk cartridge for insertion into a 130 mm disk drive apparatus. The cartridge adaptor emulates the external dimensions and features of a 130 mm cartridge so that it can be loaded into and manipulated by the 130 mm drive in the same manner as a normal 130 mm cartridge. The cartridge adaptor contains a shutter arm for opening the 90 mm cartridge door upon insertion of the cartridge into the adaptor cartridge in much the same manner as a 90 mm cartridge door is opened when the cartridge is inserted into a 90 mm drive. The cartridge adaptor also contains a pseudo 90 mm spindle located in an adaptor hub. The pseudo spindle fits within the centering hole of the 90 mm media disk. The adaptor cartridge carries an annular magnet for attracting and holding the 90 mm media disk on the adaptor hub. The underside of the adaptor hub is a simulated 130 mm cartridge hub for mating with the 130 mm drive spindle. The adaptor cartridge may also include a rotating permanent magnet directly above the 90 mm cartridge shutter opening for providing a bias magnetic field to enable writing a magneto-optic disk. Proper orientation of the rotating magnet is provided by the 130 mm drive bias coil.