On September 14, 1956, at IBM's Glendale Laboratory in Endicott, New York, IBM demonstrated the 650 RAMAC (Random Access Method of Accounting and Control) Magnetic Drum Data Processing Machine. The machine used a series of IBM 355 disk memory units. Also in September 1956 the machine was demonstrated at the U.S. Atomic Energy Commission exhibit at the Atoms for Peace Conference in Geneva.
"The addition of disk storage to the IBM 650 Magnetic Drum Data Processing Machine made possible 'single step processing.' Instead of accumulating data to be processed in stages, transactions could now be processed randomly as they occurred and every record affected by the transaction could be automatically adjusted in the same processing step. Each IBM 355 held 50 disks subdivided on each side into tracks for the storage of almost all active accounting records. Up to four IBM 355 units could be connected to the 650 system" (http://www-03.ibm.com/ibm/history/exhibits/storage/storage_355.html, accessed 10-21-2013).
The 355 disk memory unit was the first hard drive. It permitted random access to any of the million characters distributed over both sides of 50 two-foot-diameter disks. It stored about 2,000 bits of data per square inch and had a purchase price of about $10,000 per megabyte. (By 1997 the cost of storing a megabyte on a hard drive dropped to around ten cents.)