Let's say you have your PC (with files) and the external that has a copy of the files. This fails old school IT ideals about backup since backup means you have more than one copy. Wait! You say you have a copy? No, when your PC fails you only have one copy left so there is no backup copy.

This simply means you need no less than 3 copies of the files you intend to safeguard.

Back to your external hard disk. For me, the program SYNCBACK found with google does all I need and I just moved to a new machine with its help. And I have no less than 2 external hard disks, a few USB memory keys and some DVDRW media with files for good measure.

I find most owners don't become serious about backup till they suffer loss. In other words, backup might not be a teachable area but one that is learned. All I can do is share what I do and how it works for me.
Bob