I bought this magazine in Sainsbury's the other day and it has a number of free products that can be used to backup or clone a hard disk.
Computer Active Ultimate Guide - PC Maintenance and Upgrades
More here:
http://www.computeractive-direct.co.uk/index.c fm?p=16&itemid=2956
Most companies also offer trial versions of backup or clone software that you can download from their web sites.
Acronis True Image (available on the magazine above) is a good piece of software to make a complete back up of your hard disk (back up rather than clone).
IF your hard disk IS failing then if you make an Acronis backup of your MAIN disk to your external drive at least everything will be saved if the hard disk does fail on you.