A bit of overkill, but you can use the 'dd' command for this. 'dd' stands for Disk Dump, and will dump an exact duplicate, bit by bit, of a hard drive into a file. I've only used this to create CD and DVD iso files myself, so I don't know if there's anything specific when using a hard drive... But it's worth looking into.
For creating an ISO for a CD, the command is:
dd if=/dev/cdrom of=/path/to/file.iso
You can read more about dd in it's man page (man dd), and probably there's a tutorial somewhere for exactly what you want online. Here's one I found with a quick google search: http://www.linuxweblog.com/dd-image
Edit: I don't know if this is relevant or not, but CDs are generally read-only, whereas your hard drive probably is not... And probably is having data written to it even when you don't realize it. So, it's probably a good idea to run these commands from the LiveCD, and not from the hard drive you're trying to back up.