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Terabyte Territory

Brian Hayes

Painted Platters

Figure 1. The first computer disk drive . . .Click to Enlarge Image

The first disk drive was built in 1956 by IBM, as part of a business machine called RAMAC (for Random Access Method of Accounting and Control). The RAMAC drive was housed in a cabinet the size of a refrigerator and powered by a motor that could have run a small cement mixer. The core of the device was a stack of 50 aluminum platters coated on both sides with a brown film of iron oxide. The disks were two feet in diameter and turned at 1,200 rpm. A pair of pneumatically controlled read-write heads would ratchet up and down to reach a specific disk, as in a juke box; then the heads moved radially to access information at a designated position on the selected disk. Each side of each disk had 100 circular data tracks, each of which could hold 500 characters. Thus the entire drive unit had a capacity of five megabytes—barely enough nowadays for a couple of MP3 tunes.

RAMAC was designed in a small laboratory in San Jose, California, headed by Reynold B. Johnson, who has told some stories about the early days of the project. The magnetic coating on the disks was made by mixing powdered iron oxide into paint, Johnson says; it was essentially the same paint used on the Golden Gate Bridge. To produce a smooth layer, the paint was filtered through a silk stocking and then poured onto the spinning disk from a Dixie cup.

Although the silk stockings and Dixie cups are gone, the basic principles of magnetic-disk storage have changed remarkably little since the 1950s. That was the era of vacuum tubes, ferrite-core memories and punch cards, all of which have been displaced by quite different technologies. But the latest disk drives still work much like the very first ones, with read and write heads flitting over the surface of spinning platters. David A. Thompson and John S. Best of IBM write: "An engineer from the original RAMAC project of 1956 would have no problem understanding a description of a modern disk drive."

The persistence of the basic mechanism makes the quantitative progress all the more striking. Compare the RAMAC with a recent disk drive, also from IBM, called the Deskstar 120GXP. The new drive has just three platters instead of 50, and they are only three-and-a-half inches in diameter—more like coasters than platters—but in aggregate they store 120 gigabytes. Thus the surface area of the disks has shrunk by a factor of almost 800 while their information capacity has increased 24,000 times; it follows that the areal density (the number of bits per square inch) has grown by a factor of about 19 million.

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