The IBM 305 RAMAC was the first commercial computer that used a moving head hard disk drive (magnetic disk storage) for secondary storage. IBM introduced the storage unit on September 4, 1956 before unveiling the entire computer nine days later on September 13. RAMAC stood for "Random Access Method of Accounting and Control". Its design was motivated by the need to replace the punch card tub file used by most businesses at the time. The first RAMAC to be used in the US auto industry was installed at Chrysler's MOPAR Division in 1957. It replaced a huge tub file which was part of MOPAR's parts inventory control and order processing system. The 305 was one of the last vacuum tube computers that IBM built. The IBM 350 disk system stored 5 million 8-bit (7-bits plus 1 odd parity bit) characters (about 4.4 MB). It had fifty 24-inch diameter disks. Two independent access arms moved up and down to select a disk and in and out to select a recording track, all under servo control. Average time to locate a single record was 600 milliseconds. Several improved models were added in the 1950s. The IBM RAMAC 305 system with 350 disk storage leased for $3,200 per month in 1957 dollars, equivalent to a purchase price of about $160,000. More than 1000 systems were built. Production ended in 1961, the RAMAC computer became obsolete in 1962 when the IBM 1405 Disk Storage Unit for the IBM 1401 was introduced, and the 305 was withdrawn in 1969.
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