Write once read many

Write once read many (WORM) describes a data storage device in which information, once written, cannot be modified. This write protection affords the assurance that the data cannot be tampered with once it is written to the device.

On ordinary (non-WORM) data storage devices, the number of times data can be modified is limited only by the lifespan of the device, as modification involves physical changes that may cause wear to the device. The "read many" aspect is unremarkable, as modern storage devices permit unlimited reading of data once written.

WORM drives preceded the invention of the CD-R and DVD-R. An example was the IBM 3363. These drives typically used a 12 in (30 cm) disk in a cartridge, with an ablative optical layer that could be written to only once, and were often used in places like libraries that needed to store large amounts of data. Interfaces to connect these to PCs also existed.

Punched cards and paper tape are obsolete WORM media. Although any unpunched area of the medium could be punched after the first write of the medium, doing so was virtually never useful. Read-only memory (ROM) is also a WORM medium. Such memory may contain the instructions to a computer to read the operating system from another storage device such as a hard disk. The non-technical end-user, however, cannot write the ROM even once, but considers it part of the unchangeable computing platform.

WORM was utilized for Broker-dealer records within the Financial Industry Regulatory Authority and the U.S. Securities and Exchange Commission.

The CD-R and DVD-R optical disks for computers are common WORM devices. On these disks, no region of the disk can be recorded a second time. However, these disks often use a file system based on ISO 9660 that permits additional files, and even revised versions of a file by the same name, to be recorded in a different region of the disk. To the user of the disk, the disk appears to allow additions and revisions until all the disk space is used.

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