A few of my ext. drives came with image backup software. The program checks to see what has been added or changed since the last BU and copies what it needs to make a mirror of the drive being backed up. It also has a scheduler in it to BU when you are not using the computer. Works well and is forget/idiot proof. I would think there is some freeware software on sites like cnet.com that would accomplish this as well.
I personally don't care for third party backup programs because most create a proprietery file that can only be viewed or opened by the software. I prefer to back up the original files as they are in a format that can be viewed on any PC. Just my 2 cents.