From wikipedia: Recordable MiniDiscs use a magneto-optical system to write data; a laser heats one side of the disc to its Curie point, making the material in

From wikipedia: Recordable MiniDiscs use a magneto-optical system to write data; a laser heats one side of the disc to its Curie point, making the material in the disc susceptible to a magnetic field; a magnetic head on the other side of the disc alters the polarity of the heated area, recording the digital data onto the disk. Playback is accomplished with the laser alone: taking advantage of the magneto-optic Kerr effect; the player senses the polarization of the reflected light and thus interprets a 1 or a 0. Recordable MDs can be rerecorded repeatedly; Sony claims up to one million times.