I came across this interesting photo on the World Wide Web, the other day. The odd-looking wardrobe-sized cabinets, pictured sometime in the late '50s, are computer disk drives, made by IBM. They weighed over a ton, but they could store 5 megabytes of data - enough for roughly 1600 pages of A4 text, for example - which must have seemed quite a lot, back then. So must $50,000, which is what one of these would have cost you in 1956.

Things had improved a bit by 1980. By now, you could buy a 10 megabyte drive in a much more compact unit that would fit on a desk and store about 3200 pages of text. You'd only need to shell out a few thousand dollars, as you can see from the advertisement above - but that was still a sizeable quantity of cash in 1980.

A few weeks ago I bought a flash drive (essentially a hard disk, with no moving parts), very much like the one in the photo above in a music shop in Newcastle. Its capacity is 1 gigabyte (about 1000 megabytes, or 200 of the cabinets in the first picture). It's mostly made of plastic, about the same size as a disposable lighter, and designed to be used as a keyring. It cost £12, which I think would be less than $2 in 1956 money, but you can find one for less than £10 now if you shop around.

If I wanted to, I could store 320,000 A4 pages of text on it. However, I use this particular drive for storing photos from my digital camera; it holds about 2000 images. I could get roughly 10 images onto one of the IBM cabinets in the first photo, in the unlikely event that I could hook one up to one of my computers. The main, internal hard drive in my desktop PC has a capacity of 250 gigabytes, or 50,000 IBM cabinets. It would store eighty million pages of text, or half a million images from my digital camera. It's about the size of a paperback, and I expect it would cost about £70 to replace.

By the middle of this century, I expect my keyring will seem every bit as quaint and primitive as those IBM disk cabinets.