Early IBM disk storage

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The invention of magnetic disk storage, pioneered by IBM in the 1950s, was a critical component of the computer revolution. This article describes some (but not all) of the disk drives introduced by IBM from 1956 through 1993.

The basic mechanical arrangement of hard disk drives hasn't changed since the IBM 1301. Disk drive performance and characteristics are measured by the same standards now as they were in the 1950s.

This survey concludes with a modern () PC hard drive for comparison. Few products in history have enjoyed such spectacular declines in cost and size along with corresponding improvements in capacity and performance.

IBM 350

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.[1]

Simultaneously a very similar product, the "IBM 355 Random Access Memory" was announced for the IBM 650 computer system. RAMAC stood for "Random Access Method of Accounting and Control."

Its design was motivated by the need for real time accounting in business.[2] The 350 stored 5 million 7-bit (6-bits plus 1 odd parity bit) characters (about 4.4 megabytes).[3] It had fifty diameter disks with 100 recording surfaces. Each surface had 100 tracks. The disks spun at 1200 RPM. Data transfer rate was 8,800 characters per second. An access mechanism moved 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 The 350 was officially withdrawn in 1969.

The 350's cabinet was 60 inches (152 cm) long, 68 inches (172 cm) high and 29 inches (74 cm) deep. IBM had a strict rule that all its products must pass through a standard 29.5 inch (75 cm) doorway. Since the 350's platters were mounted horizontally, this rule presumably dictated the maximum diameter of the disks.

Currie Munce, research vice president for Hitachi Global Storage Technologies (which has acquired IBM's storage business), stated in a Wall Street Journal interview[4] that the RAMAC unit weighed over a ton, had to be moved around with forklifts, and was delivered via large cargo airplanes. According to Munce, the storage capacity of the drive could have been increased beyond five megabytes, but IBM's marketing department at that time was against a larger capacity drive, because they didn't know how to sell a product with more storage.

In 2002, the Magnetic Disk Heritage Center began restoration of an IBM 350 RAMAC in collaboration with Santa Clara University. In 2005, the RAMAC restoration project relocated to the Computer History Museum where efforts to restore the drive for public display continue.

IBM 353

The IBM 353 used on the IBM 7030, was similar to the IBM 1301, but with a faster transfer rate. It had a capacity of 2,097,152 (221) 64-bit words (two 32 data bit half words each with 7 ECC bits) and transferred 125,000 words per second.[5] Unlike the flying heads of the 1301, the 353 used the older head design of the IBM 350 RAMAC.

IBM 355

The IBM 355 was announced on September 14, 1956 as an addition to the popular IBM 650. It used the same mechanism as the IBM 350 and stored 6 million 7-bit decimal digits.[6] Data was transferred to and from the IBM 653 magnetic core memory, an IBM 650 option that stored just sixty 10-digit words, enough for a single sector of disk or tape data.