Here's the method I have been using successfully for some time now:

http://ubuntuforums.org/showthread.php?t=1679827&page=2

I backup the OS disc onto a USB stick. I can boot from the OS stick as well, and restore from it.

Complete backup of the OS takes about 19 minutes using the dd command.

All my data is held on another internal disc, and I use rsync to back that up onto an external USB hard disc.