I use a 1tb USB drive for storing all of my images, application installs, virtual machines, source code for development projects, schematics, etc. I try to back it up on a regular basis (approx once per month) to a couple of other USB drives that are normally offline in storage. (Yeah I know... I need to rotate the drives through the safety deposit box at bank for an offsite copy...it is on the home DR list of things to-do.)
Here is the commands to make this process work with Ubuntu 10.4.
Example: rsync -vrlptg --delete '/media/SOURCEUSB/' '/media/TARGETUSB/'
Real World: rsync -vrlptg --delete '/media/Iomega 1TB HDD/' '/media/Iomega 500mb HDD/'
Command to compare hard drives: df -h