IBM 350

IBM manufactured magnetic disk storage devices from 1956 to 2003, when it sold its hard disk drive business to Hitachi. Both the hard disk drive (HDD) and floppy disk drive (FDD) were invented by IBM and as such IBM's employees were responsible for many of the innovations in these products and their technologies. The basic mechanical arrangement of hard disk drives has not changed since the IBM 1301. Disk drive performance and characteristics are measured by the same standards now as they were in the 1950s. Few products in history have enjoyed such spectacular declines in cost and size along with equally dramatic improvements in capacity and performance.

IBM manufactured 8-inch floppy disk drives from 1969 until the mid-1980s, but did not become a significant manufacturer of smaller-sized, 5.25" or 3.5" floppy disk drives (the dimension refers to the diameter of the floppy disk, not the size of the drive). IBM always offered its magnetic disk drives for sale but did not offer them with original equipment manufacturer (OEM) terms until 1981. By 1996, IBM had stopped making hard disk drives unique to its systems and was offering all its HDDs on an OEM basis.

IBM uses many terms to describe its various magnetic disk drives, such as direct access storage device, disk file and diskette file. Here, the current industry standard terms, hard disk drive and floppy disk drive, are used.

The IBM 350 disk storage unit, the first disk drive, was announced by IBM as a component of the IBM 305 RAMAC computer system on September 13, 1956. Simultaneously 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 disk storage shipped to Zellerbach in June 1956; however, production shipment announced to begin "mid-1957" may not have occurred until as late as January 1958.

Its design was motivated by the need for real time accounting in business. The 350 stores 5 million 6-bit characters (3.75 MB). It has fifty 24-inch (610 mm) diameter disks with 100 recording surfaces. Each surface has 100 tracks. The disks spin at 1200 RPM. Data transfer rate is 8,800 characters per second. An access mechanism moves a pair of heads up and down to select a disk pair (one down surface and one up surface) and in and out to select a recording track of a surface pair. Several improved models were added in the 1950s. The IBM RAMAC 305 system with 350 disk storage leased for $3,200 per month. The 350 was officially withdrawn in 1969.

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