Magnetic drum units consist of a metal rotating cylinder coated on the outside surface with a ferromagnetic recording material and a row of read-write magnetic heads arranged along the axis of the drum. Magnetic drum units first came into use in the 1950s at the start of the computer age. Although similar to magnetic disk units, the magnetic heads of these drum units do not move, allowing for faster read and write speeds than magnetic disks. Magnetic drum units were employed as internal memory units in many early computers, and were later deployed as external memory units after the advent of magnetic core memory．
The Electrotechnical Laboratory developed the first magnetic drum unit in Japan in 1957 for the ETL Mark IV's internal memory unit. The drum had a storage capacity of 1,000 words and a rotational speed of 18,000 rpm. Hokushin Electric (later Yokogawa Hokushin Electric and today Yokogawa Electric) built the mechanical portion of the drum unit and Tokyo Telecommunications Engineering (now Sony) built the magnetic heads and their assemblies. Hokushin Electric continued to produce magnetic drums, developing high-rpm units for internal memory device applications and low-rpm large-capacity units for external memory device applications. High-rpm units were used in NEC and Hitachi computers and in Kyushu University's KT-1 Computer for Translation Experiments. Low-rpm units were used in Kintetsu Railway's seat reservation system that was developed by NEC．
Although used as main storage memory units in the 1950s, magnetic drums started to be displaced by magnetic core memories in the 1960s. Instead, magnetic drums were used as high-speed online auxiliary memory units in conjunction with magnetic tape units for mass data storage. The early generations of magnetic drum units had fixed-head assemblies. Fixed heads, however, required a gap of several microns to account for the rotational accuracy of the drum and temperature expansions. This gap limited the bit density that could be achieved on fixed-head magnetic drums. Floating heads were developed to overcome this limitation, and by the mid-1960s, Japanese computer makers had developed working floating-head drum units. NEC, in 1966, delivered a high-speed, large-capacity magnetic drum unit to Osaka University that used floating magnetic heads and metallic plating as its recording surface. In 1968, Fujitsu completed a high-performance, large-capacity magnetic drum unit that used a floating-head assembly. In 1970, Nippon Telegraph and Telephone Public Corporation (now NTT) and Hitachi jointly completed the Magnetic Drum Unit Model 1 (denoted as JS4150 and having a storage capacity of 4 megabytes), a floating-head unit that had 10 times the bit density of conventional fixed-head units. Taking advantage of its joint research with NTT, Hitachi released the H-8566 magnetic drum unit in 1968 and the H-8567 in 1969 for its HITAC-8000 series of computers．