Write Once, Read Many (alternatively Write One, Read Multiple or Write Once, Read Mostly or WORM) refers to computer data storage systems, data storage devices, and data storage media that can be written to once, but read from multiple times.
Media such as diskettes and 9 track tape, can be made read-only by enabling the write-protect tab or removing the write-enable ring, but these safeguards are easily defeated. Any media can be made read-only by physically securing it away from any drive that can write to that media. But each media is read-write until it is secured, and each media has to fill up before it becomes economical to secure it.
Software solutions are more versatile, as they can allow additional data to be written to the media until it is full, while disallowing erasure or overwriting of data already written. But they also require physical control of the media. Furthermore, the software must be audited, and computer systems must be configured to ensure that no other software can access the media.
Modern WORM systems overcome most problems by building a tested software solution into the cartridges, drives, controllers, or operating system. For removable media, the high-capacity drives are new enough that no available drive will violate the rules. Other features ensure that any data somehow overwritten in spite of restrictions will be detectable.
Storage systems which can restrict writing to one time only include: Super DLT (Super DLT II is used for both standard and WORM operations, as demanded by the user), Linear Tape-Open (LTO), AIT, and various hard disk drive-based systems. Some of these systems allow re-use of recorded media.
Punched media and PROMs are open to tampering because each bit is WORM, not the collected data. The data can be destroyed by writing every bit, or modified by changing selected unwritten bits. In punched media, all bits are 0 until punched to a 1. Holes can usually be added using a standard punch for that media. Holes cannot be filled in, or at least not nearly as easily. Punched media usually includes checksums for error detection, which also tends to detect tampering. In PROMS, all bits are usually 1 until they are burned to a 0, after which they cannot be restored to 1. Again, a checksum is the safeguard against errors and tampering.
Modern packaged WORM solutions could be bypassed by use of a rogue drive. Erasure is easy, but a magnet can do the same job. Modifying data is expected to be quite difficult, as the data are stored by possibly non-disclosed methods, or tracked by clever schemes that make rewriting even one block of data impossible to do undetectably. The strongest tool for a full medium might be copying the medium and writing a new copy to a new medium. But a separately stored checksum or an unalterable serial number on the medium would make the replacement detectable. Lastly, the data stream itself is most likely compressed AND encrypted, making it extremely difficult to modify.