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.[Note 1]


WORM drives preceded the invention of the CD-R and DVD-R. An example was the IBM 3363.[1] 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.[2]

Current WORM drives

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.

A version of the Secure Digital flash memory card exists in which the internal microprocessor does not allow rewrites of any block of the memory.

Multiple vendors beginning in the early 2000s developed Magnetic WORM devices. These archival grade storage devices utilize a variation of RAID and magnetic storage technologies to secure data from unauthorized alteration or modification at both the hardware and software levels. As the cost of magnetic (and solid state) storage has decreased, so has the cost for these archival storage technologies. These technologies are almost always integrated directly to a content/document management system that manages retention schedules and access controls, along with document level history.[3]

There are multiple vendors providing Magnetic Storage technologies including (but not limited to) NetApp,[4] EMC Centera,[5] KOM Networks,[6] and others. In 2013, GreenTec-USA, Inc. developed WORM hard disk drives in capacities of 3 TB and greater. Prevention of rewrite is done at the physical disk level and cannot be modified or overridden by the attached computer.[7][8]


In recent years there has been a renewed interest in WORM based on organic components, such as PEDOT:PSS[9][10] or other polymers such as PVK[11] or PCz.[12] Organic WORM devices, considered organic memory, could be used as memory elements for low-power RFID tags.[13]


  1. Historical exceptions include time-limited discs such as Flexplay, designed for short-term rental of movies; and early non-volatile memory technologies such as magnetic-core memory and bubble memory, from which reading data also erased it.


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