An external hard drive is a useful thing for providing temporary storage, for example when transferring files from one computer to another or to store photographs whilst you're on holiday. We do sell these drives and the price is €119.99 for 160Gb or €159.99 for 320Gb.

However, care needs to be taken when using them as a backup: A hard drive, by its nature, with many moving parts and components, has a limited lifespan and can fail if dropped, exposed to extremes of temperature, water etc. There is also a danger that you could leave the external drive connected during a storm and hence loose both the computer's own drive and the backup to an electrical surge.

In my opinion, it is better to back-up using either DVD's (if you're not backing-up a huge amount of data) or tapes if the amount of data is larger. Normally only medium-sized to large companies would require tape storage.

You can buy software which makes the whole process "much" easier. Normally, a "Full" backup (i.e. a copy of all your files, e-mails etc) is taken infrequently (maybe once per month, or every few months) and an "Incremental" backup is taken more often (e.g. daily or weekly). The incremental backup only takes a copy of files which have been created or changed since the last full backup. This strategy reduces the number of disks which you need to use and the amount of time which it takes.

You instruct the software to take a full and incremental backup and whatever intervals you choose, and it will then prompt you to insert a blank disk whenever needed. Once it's setup, you shouldn't need to do anything other than insert and remove the blank DVD's. The backup software makes all this very easy to achieve and, by backing-up to DVD, you are much less likely to loose your files since a DVD is rather difficult to damage.  

There are several different companies offering software for download from the internet (you can find them on Google).