【Hitachi】 H-8595 Magnetic Disk Unit

The H-8595 magnetic disk unit employed a lightweight head system using a fixed disk and CSS (Contact Start Stop) in order to overcome mechanical precision limits on recording media exchange and technical difficulties with dust control between the head and disk. The fixed disk and lightweight head system are used in all HDD even today, and this was an epoch-making unit. The lightweight CSS-type head was also known as a "Winchester head", and it was used for more than 10 years in subsequent disk units until the appearance of the thin-film head. The distinguishing feature of this head was that the read/write core was formed by machining ferrite to a head slider shape, forming the ring core on this part, and then winding very fine wire (a few tens of microns in diameter) around the ring core.
Using these technologies, the distance between the head and disk was dramatically reduced to about 0.5μm, and this enabled high-density recording. The fixed media system was used—thereby eliminating technical constraints due to the need to ensure media compatibility—and this kicked off the subsequent competition to achieve higher density recording and higher capacity. However, even though capacity was increased, the infinite virtual memory capacity and ease of data storage given by previous replaceable media resulted in a switch to separate media, such as tape, with the advent of this unit.
With this unit, the host side connection interface of the controller was made compatible with the IBM 3350 system, thereby enabling worldwide expansion via US and European sales companies as an IBM 3350 disk PCM (Plug Compatible Machine).

H-8595 Magnetic Disk Unit Specifications
Completion date October 1978
Memory capacity per unit 630Mbyte/DKU
Memory capacity per spindle 317Mbyte
BPI 6,250BPI
Surface recording density 3Mbit/square inch
Disk size/quantity 14-inch/8 disks
Head type/number per actuator Lightweight inductive/30+1(servo)
Average access time 20ms
Disk rotation speed 3,600rpm
Data transfer speed 1.2Mbyte/s

H-8595 Magnetic Disk Unit