With digital photos, video and MP3 collections all vying for a place on your hard drive, it's all too easy to run out of storage space.

An external hard drive with a fast USB 2.0, FireWire or eSATA connection gives you extra capacity, as well as offering an elegant back-up solution.

It also makes for an easy way to transport large files and collections of files from one computer to another, regardless of physical location.

What capacity disk drives are available?

Small, practically pocket-sized external hard drives are available with storage capacities typically of between 60GB and 320GB in size.

These are based on the 2.5-inch hard disks that are commonly used in laptop computers.

Larger 3.5-inch disk-based hard drives are also available, most commonly with storage capacities of up to 1TB (1,000GB) and even more in some cases, for larger collections of files.

Can I get a drive that doesn't need a power supply?

One of the joys of small capacity hard drives, based on laptop-style hard disks, is that they have a low power consumption. In most cases, you don't need a separate power supply for these, making portability even easier.

Often referred to as 'bus-powered', these hard drives take all the power that they need from the USB connection with which it links to the host computer.

In some cases, usually when powering the drive from some laptop computers, there isn't enough power available from a single USB port.

Small external hard drives therefore usually come with an extra USB connector, so that you can feed power from two USB ports rather than just one.

How fast is the transfer speed of external hard drives?

Most external hard drives rely on a USB 2.0 connection, which is pretty quick with a maximum speed of 480MBps.

FireWire 800 can be noticeably faster, with data speeds of 800MBps but many PCs don't feature FireWire ports as standard.

For maximum speed, the latest eSATA interface is around six times faster than USB 2.0, delivering up to 3,000MBps. Some of the latest external hard drives come with all three interfaces, making them compatible with almost any computer.

How rugged are portable hard drives?

Ultra-portable slim-line external hard drives tend to be built down to a size, rather than featuring a particularly rugged design, whereas larger drives often have built-in shock protection.

Even so, it's important not to move or jog your external hard drive while it's powered up and in use, as this can damage the read/write heads and the surface of the magnetic platters.