You can use rsync and a perl script to keep rotating snapshots with minimal duplication.
You can use rsync for the initial copy as well.
I would also suggest using a USB external drive. That way you will not have both drives corrupted by a bad controller, or simultaneously trashed by a power surge or motherboard fault.
This is what I do, and I also do a network backup. I use the grsync GUI front end for rsync for both.