WORM

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Write Once, Read Many

An optical disc storage device that uses laser technology similar to the CD-ROM. Information written to the WORM disc, cannot be altered. The advantages of WORM are increased disc density and life expectancy.^[1]

Data storage devices (e.g. CD-ROM's) where the space on the disks can only be written once. The data is permanently stored. This is often today's primary media for archival information.
The expected viable lifetime of a WORM is at least 50 years. Since it's impossible to change, the government treats it just like paper or microfilm and it is accepted in litigation and other record-keeping application.
On the negative side, there is no current standard for how WORM's are written. The only ISO standard is for the 14" version, manufactured only by one vendor. A 5.25" standard is emerging from the European Computer Manufacturing Association but is not yet accepted. Further, WORM discs are written on both sides, but there are currently no drives that read both sides at the same time. As for speed, WORM is faster than tape or CD-ROM, but slower than magnetic. Typical disk access times run between 40 and 150 milliseconds (compared with 11 ms for fast magnetic disks and 300 ms for CD-ROM. Data transfer rates run between 1 and 2 MB/sec (compared with 5 to 10 for magnetic discs and 600KB/sec for CD-ROM.
Disk sizes run from 5.25" (1.3 gigabytes) to 12" (8 to 10 gigabytes) capacities. There is also a 14'" disc (13 to 15 gigabytes), only manufactured by Kodak's optical storage group. WORM's can also be configured into jukeboxes. There are various technologies:

Technology Description	Benefit	Drawback
Ablative: Laser burns holes in disk	Unalterable data	Dust, moisture may affect media
Bubble-forming: Laser forms bubbles in the media	Unalterable data	Few drives available
Dye Polymer: Laser heats dyed layer to form bumps	Potential low media cost	Laser mechanism more expensive; disks wear out faster; few drives available
Magneto: Laser focuses magnetic field	Many suppliers, long disk life	No true WORM in multi-function; data theoretically alterable
Phase change: Laser heat changes disk's molecular structure	One-pass data (no erase step)	Same as Dye Polymer

From Imaging Magazine, September, 1994^[2]

Footnotes

^ Legal Electronic Document Institute, Basic Principles of Automated Litigation Support (2005).
^ American Document Management, Glossary of Terms, http://www.amdoc.com/glossary.shtml.