Lab

500GB Hard disks

When you bought your PC, its hard disk probably seemed so huge it would be impossible ever to fill it. Yet, within a few years, you've somehow achieved it. The hard disk's stuffed. You need more storage.

What's more, some of those files you've accrued are downloads that it would be hard if not impossible to find again. Others are your own files, including Word documents, financial records, pictures and videos. If your hard disk were to go wrong, the loss would be incalculable. You need a backup drive.

The easy answer to either or both of these problems is an external desktop hard disk. Installation is a simple matter of plugging in a couple of cables, and you get the added advantage of being able to carry your data around with you. If you actually want to carry it around on a regular basis, you should go for a portable drive, rather than a desktop unit: these are smaller and lighter, and don't need a separate power supply. But that portability comes at a price, while desktop drives are far more affordable. The units we're looking at this month all have a huge 500GB capacity, yet most come in under £100.

So which one should you buy? You might think all hard disks are alike, and it's true that all seven of this month's contenders will do essentially the same job. From that point of view, you might just want to plump for the cheapest - and as it happens, the cheapest is a very nice drive. But there are differentiating factors. Speed is one: as you'll see from our test results, an operation that takes 40 seconds with some drives can take a minute with others. It's not a huge gap, but those seconds add up.

If speed's a concern, you can get vastly better performance if your PC has an eSATA interface, as long as you choose a drive that supports it.

Another issue to consider is the warranty. Most of the drives on test come with an already impressive two-year warranty, but some are more generous, with Seagate and Maxtor offering a remarkable five years. Of course, no one can replace your precious data, but you may be reassured to know that the drive itself is covered - and the maker's willingness to offer a guarantee says something about the drive's likely longevity.

Finally, you may also want to consider looks. Is the drive supermodel sleek or the aesthetic equivalent of a bin man in a string vest? Will it dominate your workspace or nestle unobtrusively? These may sound like trivial questions, but if you're buying a device that's going to sit on your desk for the foreseeable future, you want one that will enhance your space rather than just cluttering it.

On the coming pages, we pit external hard disks from all seven of the major manufacturers against one another. Read on for our verdicts on speed, ergonomics, features and price.