Well, I've decided to format my box and before I do this, I (obviously) want to back everything up to a secondary hard drive... Is there an easy, secure, guarantee way to do this with cPanel, but rather than a separate server - back up to another hard drive?

WHM has this feature. I suppose you have WHM if you have Cpanel.

just choose the partition in your WHM (logged as root) in your backup section
Make sure you enable backup , daily, or you can run it directly from ssh later by typing /scripts/cpbackup
It will backup all your users to the secondary hdd.