The first hard disk drive on the planet, the r.a.m.a.c (random access memory for accounting and control), was made by IBM in the nineteen fifties. It was as big as a large wardrobe and could contain a capacity of five whole megabytes! The archaic machine cost about fifty thousand US dollars at the time. The price now per gigabyte of memory when talking about hard disk drives less than a dollar, or 1 ten-millionth of what it was sixty years ago. This development rate has proven faster than that of telecommunications data transfer rates and of semi-conductor density increases.

Hard disk drives store their sequence of data bits in linear tracks, which follow around a magnetic disk in concentric circles. In order to fit more data over a smaller area, the gaps between the data tracks must reduce, as well as the spacing between the bits. These dimension quantities are measured in bits per inch (in a circumference-wise direction), and tracks per inch (in a radius-wise direction). These get smaller as the technology gets better; also, one must take into account the tiny areas or gaps between tracks that are there to compensate for a slightly misaligned reader head, so it doesn't read the neighboring tracks.

The method of measuring the total quantity of data storable over a surface area is quantity per square inch, these days, gigabits per square inch. This measurement is a combination of your bits per inch and tracks per inch. It is known as the areal density. This density has, since that first wardrobe hard drive, had a steady increase at 30 percent per year, and from the early nineties when magneto head resistance was brought about, jumped to 60. Then again in the late nineties, it leaped up to 100 percent yearly increase with the introduction of giant magnet resistance head technology. Now though, the rate of growth has slowed to under 60 percent.