CD-MO
Prior to the introduction of the CD-RW technology, a standard for magneto-optical recordable and erasable CDs called CD-MO was introduced in 1990 and set in the Orange Book, part 1, and was basically a CD with a magneto-optical recording layer. The CD-MO standard also allowed for an optional non-erasable zone on the disk, which could be read by normal CD-ROM reader units.
Data recording (and erasing) was achieved by heating the magneto-optical layer's material (e.g. DyFeCo or less often TbFeCo or GdFeCo) up to its Curie point thus erasing all previous data and then using a magnetic field to write the new data, in a manner essentially identical to Sony's MiniDisc and other magneto-optical formats. Reading of the discs relied on the Kerr effect. This was also the first major flaw of this format: it could be read in only special drives and was physically incompatible with non magneto-optical enabled drives, in a much more radical way than the later CD-RWs.
The format was never released commercially, mostly because of its inherent incompatibility with standard CD reading units. A similar situation was also present for early CD-R media, which suffered from either physical or logical incompatibilities.
Since the CD-MO was otherwise physically identical to "normal" CDs, it still adopted their spiral-groove recording scheme, which would have rendered it hard to use as a removable medium for repeated, small scale deletions and recordings (not unlike CD-RW). There were (and are) however some magneto-optical drives and media with the same form factor that don't have this limitation. Unlike modern CD-RWs, CD-MO allowed for hybrid disks containing both an unmodifiable, pressed section, readable in standard drives and a writable MO section.
This early introduction along with the lack of standards for disk recording software, file systems and formats, physical incompatibility as well as the introduction of the more economical CD-R disks essentially caused the format to be abandoned before commercialization, and the whole idea of a rewritable CD medium to be almost forgotten until modern phase change CD-RWs appeared. Other kinds of magneto-optical media, unbound by the limitations of the typical CD-ROM filesystems, took the place intended for CD-MO.
Read more about this topic: CD-RW