In the history of auxiliary memory devices using magnetic media, magnetic disk units were developed after magnetic drum units. Magnetic disk units had considerably higher recording densities per unit volume and, thus, were widely used as large-capacity external storage units．
Magnetic disks offered much more capacity per unit volume by stacking several rotating recordable disks. They were developed in response to mainframe systems' demands for online files that were larger than the capacity of magnetic drums. The first practical magnetic disk unit was IBM's RAMAC in 1957 with a capacity of 5 mega-characters on fifty 24-inch disks. Magnetic disks were quickly adopted after this in Japan as well. In 1963, IBM developed the IBM 1311 Disk Storage Drive, which had a capacity of 2 mega-characters and consisted of six 14-inch disks in the form of a removable disk pack. Following this, IBM came out with a series of magnetic disk units and disk packs, including the IBM 2311 (7.25 megabytes, six 14-inch disks) in 1964, the IBM 2314 (29.2 megabytes, eleven 14-inch disks) in 1966, and the IBM 3330 (100 megabytes, eleven 14-inch disks) in 1971. Since IBM had taken the market lead, the market formed around IBM-compatible devices and other manufacturers followed IBM products．
The Ministry of International Trade and Industry, seeking to boost Japan's domestic technological capabilities, made R&D into technologies for very-high-performance computers one of its major projects in 1966. As part of this project, Hitachi worked on developing large-capacity magnetic disks. In 1971, the company completed a prototype of a magnetic disk unit with a higher recording density than IBM's latest model at the time, the 3330. However, the unit never reached mass production as a commercial product．
The IBM 3340 (commonly called the Winchester) hit the market in 1973 with a fixed head-disk assembly that became the standard disk design in the industry. In 1976, IBM started shipping the 3350 (318 megabytes), which had non-removable disk packs. Hitachi, NEC, and Fujitsu each developed magnetic disk units with a storage capacity of 600 megabytes, twice the capacity of the 3350, around 1980. The Nippon Telegraph and Telephone Public Corporation, in cooperation with NEC, Hitachi, and Fujitsu, successfully developed the Model 801 (JS4370) magnetic disk unit in 1979 with 800 megabytes of storage capacity per spindle, which was the largest capacity in the world at the time. Moving into the 1980s, Japanese companies came to the fore with products using original Japanese technology．
Thin-film head technology arrived in the 1980s. NEC rolled out the N7761 (2.68 gigabytes) in 1982, a fixed disk unit that was the first Japanese-made unit with a thin-film head. The Nippon Telegraph and Telephone Public Corporation developed, in 1982, the JS4380 (commonly known as PATTY for packaged air-tight tiny disk), which used a thin-film media and achieved a recording density of 15.5 megabits per square inch, the highest ever at the time. Both PATTY and Fujitsu's F6421 (released in 1981 and known as the Eagle) improved reliability by using a completely sealed construction．
Eight-inch disks started to be manufactured in 1979 and 5.25-inch disks in 1980. Both formats were used in minicomputers and office computers. In 1979, Fujitsu completed the M2301 magnetic disk unit, which used 8-inch magnetic disks. Mitsubishi Electric, in 1981, announced the M4863, a 5.25-inch magnetic disk unit. The 3.5-inch disk became the standard for personal computers after Fujitsu completed the 3.5-inch M2225 magnetic disk unit in 1987 and Toshiba started shipping the 3.5-inch MK-130FA series of magnetic disk units in 1988. Later, 2.5-inch disks appeared for notebook computers. Toshiba came out with the 2.5-inch MK1122FC in 1990, a magnetic disk unit that used glass-substrate disks．
In 1966, IBM shipped the 2314 Disk Access Storage Facility, which contained multiple disk units to boost performance and reliability. In a similar vein, Fujitsu completed the FACOM 472K and Hitachi the H-8577 in 1970. As time went on, more and more features were incorporated in disk controllers. For example, Memorex announced the first disk cache function in 1978 and NEC followed suit in 1980．
In the 1980s, methods of processing small disks in parallel became feasible. In 1987, David Patterson at the University of California put forward the idea of Redundant Array of Inexpensive Disks (RAID). EMC, in the United States, announced a disk array for mainframes in 1990. In Japan, NEC was the first to manufacture a unit with eight parallel disks without parity (RAID 0) as a high-performance disk array for supercomputers. This was followed by Fujitsu, which started shipping a RAID 3 disk array for supercomputers in 1991. Hitachi announced the A-6511 disk array for mid-sized systems in 1992．