Ferranti Mark 1, the world's first commercially available general

Early electronic computers such as Colossus made use of punched tape, a long strip of paper on which data was represented by a series of holes, a technology now obsolete.^[15] Electronic data storage, which is used in modern computers, dates from World War II, when a form of delay line memory was developed to remove the clutter from radar signals, the first practical application of which was the mercury delay line.^[16]The first random-access digital storage device was the Williams tube, based on a standard cathode ray tube,^[17] but the information stored in it and delay line memory was volatile in that it had to be continuously refreshed, and thus was lost once power was removed. The earliest form of non-volatile computer storage was the magnetic drum, invented in 1932^[18] and used in the Ferranti Mark 1, the world's first commercially available general-purpose electronic computer.^[19]

IBM introduced the first hard disk drive in 1956, as a component of their 305 RAMAC computer system.^[20] Most digital data today is still stored magnetically on hard disks, or optically on media such as CD-ROMs.^[21] Until 2002 most information was stored on analog devices, but that year digital storage capacity exceeded analog for the first time. As of 2007 almost 94% of the data stored worldwide was held digitally:^[22] 52% on hard disks, 28% on optical devices and 11% on digital magnetic tape. It has been estimated that the worldwide capacity to store information on electronic devices grew from less than 3 exabytes in 1986 to 295 exabytes in 2007,^[23] doubling roughly every 3 years.^[24]