Holodisk

Tape Holography Technology tapes, commonly referred to as holodisks or holotapes, are a high density storage medium developed by Wattz Electronics that became the nationwide standard by 2077.

Characteristics
 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. 

Gameplay
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 and Fallout: New Vegas as data carriers of all sorts.

Behind the scenes

 * Holotapes seem to be based on a real-life recording and playback system, the 1969 "RCA SelectaVision HoloTape Player," which involved etching holographic grooves onto a tape for later playback.