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, holodisks are a high-density, laser-readable, manufactured-crystal storage device produced by Wattz Electronics. Each holodisk is capable of storing their information safely for extended periods of time, without causing read errors or data corruption. The THT reels are usually stored inside a resistant, stainless casing outfitted with a universal I/O port for connectivity. 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.

The development of technology progressed rapidly and while early holotape models could store only as much as 256 kilobytes of data (though typically, they could only hold between 128 and 64 kilobytes), the latest models from 2077 could store up to 4 terabytes (4,000 gigabytes) of data, but are far less common than holotapes due to their short time frame of availability. Storage in holotapes would be divided into "blocks" and a typical commercial holotape would have a capacity of 256 blocks to properly store data, however, the amount of data in a holotape all depended on the compression of the data.

However, holotapes still managed to take the world by storm and formed the basis of nearly every data storage setup, including transmissions, software backups, and even storing an abridged version of the Library of Congress in G.E.C.K.s. Of course, holodisks and tapes had many more uses before the war besides just data storage. Specific models were meant to hold entertainment, such as music, movies, and video games, with special-purpose players developed for displaying large, high resolution volumetric images or to play holotapes for personal use in a home environment or public setting. 

Characteristics
Holodisks are used to upload and download information from computers. While they are relatively rare quest-related items in Fallout, Fallout 2 and Fallout Tactics, they are very common in Fallout 3, Fallout: New Vegas, Fallout 4, and Fallout 76 as data carriers of all sorts. Notably, the Creation engine allowed holodisks to use terminal functionality, and added the ability to play holodisks on stationary terminals, together with interactive games.

Their appearance also varies, sometimes wildly, between games. In Fallout, its sequel, and Tactics holodisks are interchangeably referred to as holotapes and indeed contain 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 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 and Fallout 76, 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). When played, the game will show an animation of it being inserted into the PIP-Boy.

Behind the scenes

 * The design of holodisks in Fallout and Fallout 2 is different from the design of holotapes in Fallout 3 and Fallout: New Vegas, as well as those present in Fallout 4 and Fallout 76. 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). This is further expanded upon when Project SIPHON at Sugar Grove tested a high-storage, yet highly expensive, iridium-infused magnetic tape holotape for data compression up to 200 blocks.
 * Holotape was an actual invention: They were used in the 1969 RCA SelectaVision HoloTape Player, which involved etching holographic grooves onto a tape for later playback.