CDRW

CD-RW (Compact Disc-ReWritable) is a digital optical disc storage format. A CD-RW disc is a compact disc that can be written, read arbitrarily many times, erased, and written again. The technology was introduced in 1997.

CD-RW discs (CD-RWs) require readers that have more sensitive laser optics than are required to read plain CDs. Consequently, CD-RWs cannot be read in many CD readers built prior to the introduction of CD-RW. CD-ROM drives that bear a "MultiRead" certification claim compatibility.

CD-RW discs need to be blanked before reuse. Different blanking methods can be used, including "full" blanking in which the entire surface of the disc is cleared, and "fast" blanking in which only metadata areas are cleared: PMA, and pregap, comprising a few percent of the disc. Fast blanking is much quicker, and is usually sufficient to allow rewriting the disc. Full blanking removes traces of the former data, often for confidentiality reasons. It may be possible to recover data from full-blanked CD-RWs with specialty data recovery equipment; however, this is generally not used except by government agencies due to cost.

CD-RWs also have a shorter rewriting cycles life (ca. 1,000) compared to virtually all of the previously exposed types storage of media (typically well above 10,000 or even 100,000), something which is less of a drawback considering that CD-RWs are usually written and erased in their totality, and not with repeated small scale changes, so normally wear leveling is not an issue.

Their ideal usage field is in the creation of test discs, temporary short or mid-term backups, and in general, where an intermediate solution between online and offline storage schemes is required.

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 disc, 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.

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