A holodisk, holotape or THT Tape, is a durable laser-readable storage device produced by Wattz Electronics, based on the Tape Holography technology, storing their information safely for extended periods of time.
While according to the Vault Dweller's Survival Guide holotapes could hold as much as 256 kilobytes (with the middle models holding 128 kilobytes and standard tape holding only 64 kilobytes), according to Vree, high-density, manufactured-crystal holodisks could hold as much as 4,000 gigabytes (nearly 4 terabytes) of information. It is possible that the pre-War military and/or Brotherhood of Steel had access to disks superior to the commercially available tapes.
The design of holodisks in Fallout and Fallout 2 is different from the design of holotapes in Fallout 3. It is possible that more holodisk/holotape standards were developed and commonly in use before the Great War. For instance, even the 4000 GB version mentioned in Fallout would be insufficient to copy an entire library's archives (media archives holotape).
Holodisks can be used either in stationary computers or in Pip-Boys. In Fallout 3 and Fallout: New Vegas they can only be used with the Pip-Boy.