Quantum DLT 2000 Handbook page 133

Write once, read many (WORM) drives have been available for
many years and are most often used to provide a permanent record
for critical information, or are used in optical jukeboxes in hierar-
chical storage management (HSM) systems. Law firms, for
instance, often use WORM drives to store important documents
and court records. WORM media has a shelf life of at least 30
years. A standard CD format version of the WORM drive is known
as a CD-R for CD recordable. These systems store information in
standard CD format with a standard density of 650MB and can be
read by the CD drives found in most of today's PCs.
Standalone WORM drives have virtually disappeared from the
market with the introduction of multi-function drives that can act
as either a rewritable drive or a WORM drive. By using the correct
media, you can create a WORM disk that offers a permanent
record, or use rewritable media, and the same drive can write, read
and overwrite.
The most popular multi-function drive is the phase-change dual
(PD) drive. This drive uses a high intensity laser to change the crys-
taline structure on the surface of a 5.25" PD disk. The change in
reflectivity caused by the laser represents either a zero or a one. A
medium-intensity laser is then used to restore the crystaline struc-
ture so that it can be written to again. When a WORM disk is used
in the same drive, the data written to it by the laser is permanent.
Because these drives adhere to the same format used in the CD
drive found on most computers, they are limited to 650MB of
storage. This limitation places them at the low end in terms of a
standalone data backup solution. In addition, PD drives have very
slow access times. One advantage to PD drives is that they can read
standard CD-ROMs.
QUANTUM DLTtape HANDBOOK 11.11