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.
Holotapes, according to the Vault Dweller's Survival Guide, could hold as much as 256 kilobytes (with the middle models holding 128 kilobytes and standard tape holding only 64 kilobytes), while according to Vree, high-density, manufactured-crystal holodisks could hold as much as 4,000 gigabytes (nearly 4 terabytes) of information.
The design of holodisks in Fallout and Fallout 2 is different from the design of holotapes in Fallout 3 and Fallout: New Vegas. This suggests 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 unless highly compressed (Media archives holotape).
In Fallout 1, 2, and Tactics, holodisks appear to be bulky, rectangular objects lacking any sort of protective casing, giving them an appearance comparable to a VHS tape or computer hard drive. In Fallout 3 and New Vegas, they are flatter, with a gray metal casing, a vision slit that reveals the disk's tape reels, and shaped like squares about the size of a person's hand. In Fallout 4, holodisks were redesigned with an orange and yellow plastic casing that completely conceals the tape reels, with a gray metal circle in the upper left corner of the disk (Possibly for reading the stored data). The F4 holodisks are also both small enough to be fitted into the Pip-Boy 3000 and contain enough storage space to hold interactive games.