Holotape

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.

Background
Developed using tape holography technology as early as 2063, holotapes are a high-density, laser-readable, manufactured-crystal storage device. These disks and tapes were the very latest in pre-War information storage technology and quickly became the most widespread data storage medium, available in a variety of compatible variants.

While early holotape models could store only as much as 256 kilobytes of data, averaging between 128 and 64 kilobytes, the latest models from 2077 could store up to 4 terabytes (4,000 gigabytes) of data. Storage in holotapes were divided into "blocks" and a typical commercial holotape had a capacity of 256 blocks to properly store data. The amount of data in a holotape all depended on the compression of the data.

By 2287, The Institute had holotapes of their own, capable of storing the entirety of libraries, such as the Boston Public Library, onto their storage banks. Holotapes were capable of storing transmissions and software backups, and stored an abridged version of the Library of Congress within the G.E.C.K.s.

Holotapes had more uses before the war aside from data storage. Specific models were meant to hold entertainment, such as music, movies, and holotape games, with special-purpose players developed for displaying large, high resolution volumetric images or to play holotapes for personal use in a home environment via holotape players. Holotapes could utilize terminal functionality, adding the ability for use with stationary terminals, together with interactive games. 

Characteristics
Their appearance varies between games. In Fallout, Fallout 2, and Fallout Tactics, holodisks are interchangeably referred to as holotapes and contained holotape reels in a translucent protective casing, roughly comparable to a VHS tape or computer hard drive.

In Fallout 3 and New Vegas, they are flat, with a gray metal casing, a vision slit that reveals the tape reels, and square shaped. In Fallout 4 and Fallout 76, holotapes 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. When played, it can be seen inserted into the Pip-Boy.

Behind the scenes
Holotapes are based on the real world 1969 RCA SelectaVision HoloTape Player, which involved etching holographic grooves onto a tape for later playback.