Machines that count: the hard drive

Computer historian John Pratt, curator of the Machines that Count exhibition at Auckland's Museum of Transport and Technology, looks at the development of the hard drive

Spare a thought for your hard drive. It is one of the most overlooked miracles of computing, but without the hard drive there would be no personal computer. Every time you start up your PC or start an application, or open, edit or save a document, you are accessing the storage in your machine.

In a modern hard-disk drive, the heads are travelling as fast as 80 km/hr, looking for tiny segments of data no wider than 1/100th the width of a human hair. A modern hard drive can find any data on the disk in 3.5 milliseconds — that’s about the time it takes an object to fall 25mm.

That’s like finding a needle in a haystack first time, every time.

It wasn’t always the case. The first practical magnetic storage was actually a magnetically coated drum. It was developed by Engineering Research Associates, which then become part of Remington Rand and then Control Data Corporation.

The ERA drum had discrete circular tracks around its whole circumference, each with its own read/write head. ERA sold prototype systems for US$5,000 apiece and licensed the technology to anyone. Drum storage was made by IBM, for whom ERA made heads, and also by Remington Rand UNIVAC.

It was IBM that made the leap from drums to magnetic disks, in about 1951. The first drum storage system, developed at IBM’s San Jose lab, consisted of a stack of 50 24-inch disks, with a single head that was inserted over the appropriate disk. Christened RAMAC, for Random Access Method of Accounting, the system was sold with IBM’s Model 650 calculator, enabling iterative calculation for the first time.

Inside RAMAC, the heads were statically positioned on the disk by an electro-hydraulic actuator. A small compressor supplied air to keep the heads suspended 0.020 inches above the disk.

One of the first commercial disk systems was IBM’s 2314 system, introduced for the System/360. Its four cabinets contained nine disk drives, each with 11-disk platters. The whole thing was good for just 230MB of storage.

The seminal development in hard disk technology was IBM’s “Winchester” disk, which was launched in 1973. Inside the Winchester, the heads flew in the fast-moving layer of air that the spinning disk created just above the surface.

They were positioned by a “voice coil”, suspended loudspeaker-style between the poles of a magnet. A servo-mechanism located the heads on the disk and this analogue mechanism was so accurate it could compensate for small amounts of run-out or wear in the disk mechanism. When the disk was stationary the heads were located on an unused portion of the disk surface.

Every hard disk manufactured today, from the one-inch micro-drives to the biggest, most dense hard drives, is visibly descended from the Winchester drive. But, although the architecture of the hard drive hasn’t changed, performance has improved greatly. Hard disks have probably evolved even faster than processors. Disk storage density, measured as bits-per-square-centimetre, has, literally, improved billions of times since 1973.

At one point, researchers realised they needed about 100 grains of iron oxide to safely record and read a single bit, and the physical size of the oxide particle was thought to represent a ceiling on disk density until someone discovered perpendicular recording, standing the requisite grains on-end to save disk space. Whenever it seems like the days of the hard drive are numbered, along comes a new innovation and the whole cycle begins again.

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