the first commercial hard disk storage system. Dating back to 1956, the IBM 305 RAMAC (Random Access Method of Accounting and Control) complemented the IBM 350 computer, and it provided 5 million characters of storage (by the way, those were 7 bit characters, not 8 bit). RAMAC was developed in San Jos´, California. Its control unit was based on thermionic valves (remember that we are in the Fifties).

One peculiarity of the RAMAC was that it had an externally pressurized air bearing, that's to say that the read/write head flew on the disk surface like a hovercraft. The technology was superseded by the self-acting air bearing, that needs no external air supply.< The RAMAC needed an air compressor and it was an enormous device, costing 175.000 USD (but the typical agreement was to lease it for 3200 USD per month), as tall as a person, and containing 50 24" disk platters. The disks were visible in a glass walled part of the unit. The RAMAC had only two read-write heads mounted at the end of an arm; the assembly would travel up and down, and then rotate into the stack of platters to acces the desired track/platter combination.

The engineers had to solve complex problems, many of the related to the reliable maintenance of head surface distance; if the head is too far from the disk the magnetic field becomes too weak. If the head is too close, it can touch the disk surface and scratch it, which leads to data loss (see head crash).

IBM continued to use the "RAMAC" name well into the Nineties for mainframe disk arrays.

references: IBM Almadena Research Center website and Computers: an Illustrated History.
see also: Jurph's extensive hard disk WU.