The IBM Random Access Memory Accounting System was not particularly special in itself — it was built for general accounting rather a specific mission. It could record transactions in real time and provide access to any of of them from random query, and IBM had nearly 1,000 of them built over a five year period. But one component of the RAMAC 305 was in fact special — the world’s first commercially available magnetic disk storage.

On this day, September 4, in 1956 the IBM RAMAC, the first commercial computer with magnetic disk, storage was introduced.

At a length of five feet and width of about five and half, the 305’s Disk Storage Unit jutted out 29 inches from the wall. Despite its considerable heft, it was a relatively simple machine: a magnetic disk memory unit made up of 50 disks with its access mechanism and attendant mechanical and electronic controls. There was also small air compressor, and that’s it. Each of the disks had 1,000 sectors, with 100 alphanumeric characters to each sector, for a grand total capacity of 5 million characters.