Magneto-optical drive
The magneto-optical drive was a brief attempt to sell people on the idea of re-writable cd-roms before such technology was practical or available. Sealed in a dust-proof plastic case, the "cd" was actually an odd mix of magnetic material sealed beneath a plastic coating. Each write-cycle required both a pass for the laser to heat the surface, and another pass for the magnet to write the information. Suffice it to say it took twice as long to save any data vs the retrieval time. The NeXT computer was the first to offer this technology, but Canon eventually provided it to other customers. Aside from being slow, the technology also sufferred from the same price problems that plaugued other removable technologies at the time like Syquest et-al.Media for this technology is often called Floptical media.
At the Consumer Electronics Show in January 2004, Sony revealed a 1 gigabyte capacity MiniDisc. This quintupling of capacity is done using a magneto-optical trick -- to record a standard MiniDisc, a writer uses a infrared laser to heat a spot of ferromagnetic material on the disc to above its Curie point, then, it is magnetised by a recording head as it cools.
A high capacity MiniDisc uses magnetic tracks (sectors? A single bit of data) that are 5 times smaller than those used in standard MiniDiscs. By employing three magnetic layers, when a high capacity MiniDisc is read, a magnetic track swells a readable size. Specifically, the three layers are, from read-face to print-face, a displacement layer, a switching layer, and a memory layer. When it isn't being read, the magnetic field in the memory layer is the same as those in the displacement and switching layers. When a laser shines on a track, the switching layer, which has a lower Curie point than the other layers, demagnetises. It decouples from the displacement layer, whose "magnetic fence" around the track weakens, temporarily causing the track to swell to a readable size.