1956 – IBM 350A, shipment of prototype disk drive to Zellerbach, SF CA, USA; 1957 – IBM 350, first production disk drive, 5 million characters (6-bit), equivalent to 3.75 megabytes. 1961 – IBM 1301 Disk Storage Unit introduced with one head per surface and aerodynamic flying heads, 28 million characters (6-bit) per module.