1. Those that physically can be written only once. (examples of such storage media are CD-R, DVD-R, and Blu-Ray Discs) This output of data to a piece of removable WORM media not only prevents 're-working' or 'adding to' original files but also meets many of the strict compliance measures in effect by regulatory agencies, government entities, and good business practices. Since this type of WORM media is physically portable, they are often kept 'off site' to meet stringent Disaster Recovery Plans. When kept 'on site' it is most generally held in Optical Libraries which enable the data volumes to not only be archived, but also readily accessible through electronic retrieval back to the working network.

2. Stationary Hard-Drive media that enables WORM capability by using electronic keys or other measures to prevent rewriting. (Standard compliance on HDD) The reasoning behind this artificial hampering of technical capability in the latter case can be found in regulatory authorities requiring certain minimum data archival standards where information has to be reliably available on network servers, yet not allow tampering with the original files.

Optical WORM is the Clear Choice for any archival implementation. Today's magnetic disk or tape archival solutions try to emulate optical WORM technology, however unlike the implementation of an optical archive, all of these magnetic based archival solutions are proprietary, in that they require custom API's and utilize proprietary formats. In addition, magnetic disk based systems are non-removable limiting off-site storage capability. If you consider WORM tape, which is a magnetic based media, the tape cartridge can easily be erased with an external erasing tool. As with any magnetic tape this fails the test of being TRUE WORM which requires the media to be non-alterable and non-erasable. Additionally, with tape being a sequential storage device, it is not ideally suited for compliancy applications as these applications are typically database driven requiring random access storage which is what optical delivers.
On WORM DVD or Blu-Ray media all kinds of files may be stored revision-secure and in accordance with most all existing compliance and regulatory requirements. Therefore True Optical WORM is the archive solution of choice for industries with fixed content storage assignments. This may include Broadcasting, (Regardless of A/V source) Surveillance Video, Educational Material, Medical Records and Images, Financial Data and Transactions, and Production or Multimedia Files.