History of IBM magnetic disk drives

Lore & Background

The IBM 350 disk storage unit, the first disk drive, was announced as a component of the IBM 305 RAMAC computer system on September 14, 1956. A very similar product, the IBM 355, was announced for the IBM 650 RAMAC computer system. RAMAC stood for 'Random Access Method of Accounting and Control'. The first engineering prototype 350 shipped to Zellerbach Paper Company, San Francisco, in June 1956, with production shipment beginning in November 1957 to United Airlines in Denver, Colorado. Its design was motivated by the need for real time accounting in business. The 350 stores 5 million 6-bit characters on fifty-two 24-inch disks, of which 100 recording surfaces are used. Each surface has 100 tracks. An access mechanism moves a pair of heads up and down to select a disk pair and in and out to select a track. The 350 was officially withdrawn in 1969.

Later models included the IBM 1301, announced June 2, 1961, which stored 28 million characters per module using 25 large disks and 40 user recording surfaces, with 250 tracks per surface. The 1301 was the first disk drive to use heads that are aerodynamically designed to fly over the surface on a thin layer of air, allowing them to be much closer to the recording surface. It also introduced a separate arm and head for each recording surface, with all arms moving together like a comb, eliminating the time needed to move the head between disks. Maximum access time was reduced to 180 milliseconds. The IBM 1302, introduced in September 1963, quadrupled capacity over the 1301 to 117 million 6-bit characters per module.

IBM manufactured 8-inch floppy disk drives from 1969 until the mid-1980s but did not become a significant manufacturer of 5.25- or 3.5-inch floppy disk drives. IBM always offered its magnetic disk drives for sale but did not offer them with OEM terms until 1981. By 1996, IBM had stopped making hard disk drives unique to its systems and was offering all its HDDs as an OEM.

Reader's Guide

The significance of IBM's magnetic disk drives lies in their foundational role in the computer storage industry. The IBM 350, the first disk drive, introduced random-access storage to business computing, replacing sequential tape and punch card systems. Its design—spinning disks with movable read/write heads—established the basic architecture that persisted for decades. The IBM 1301's innovations, particularly the use of aerodynamic flying heads and a dedicated arm per surface, set the mechanical arrangement that has not changed since. These advances dramatically reduced access times and increased reliability. The spectacular declines in cost and physical size, along with equally dramatic improvements in capacity and performance, characterized the entire product line. IBM's decision to sell its hard disk drive business to Hitachi in 2003 marked the end of its direct involvement in manufacturing, but the technologies and standards it established—including the measurement standards still used today—remain the foundation of modern disk storage. The RAMAC 350 Disk File was designated an International Historic Landmark by the American Society of Mechanical Engineers in 1984, and a restored unit is demonstrated at the Computer History Museum.

Did You Know?

• The IBM 350 disk drive was initially cancelled by the IBM Board of Directors because of its threat to the IBM punch card business, but development continued until approved by IBM's president.
• The IBM 350 cabinet is 60 inches long, 68 inches high, and 29 inches wide, and weighs about one ton, requiring forklifts to move.
• IBM's marketing department was against a larger capacity drive for the 350 because they did not know how to sell a product with more storage.