WORM stands for white-once, read-many. WORM disks allow users to create their own CD-ROM disks by using a CD-recordable (CD-R) drive attached to a computer as a regular peripheral device. WORM disks look like standard CD-ROM disks, are available blank in market, and user can encode on it using a CD-R drive. We can use any ordinary CD-ROM drive to read information recorded on a WORM disk by a CD-R drive. As the name implies, we can write data only once on a WORM disk, but can read it many times. However, we can write all desired data on a WORM disk in multiple recording sessions. The sessions after the first one are always additive and cannot alter the etched/burned information of earlier sessions. WORM-disk-storage mechanism enables hiding of information added in a session in a subsequent session by creating File Allocation Table (FAT) at a new location, but the etchings on the surface are permanent (not removable). Such disk is called Multi- Session Disk. It uses laser beam technology (discussed earlier) for data recording/reading.
Because of its permanent recording nature and large storage capacity, WORM disk is the preferred storage medium for archival applications, which now depend on microfilm, magnetic, or paper file media. Data archiving enables recording of old data on a secondary storage medium (such as a WORM disks) so that they can be deleted from the on-line storage device (such as a hard disk), thus freeing space for new data. WORM disk is also an ideal medium for making a permanent record of data. For example, many banks use them for storing their daily transactions. They write the transactions to a WORM disk, which becomes a permanent record for them (they can only read it whenever required, but can never alter).