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.
I am looking for a good, but simple, external hard drive with an installed back up system. I have seen some that back up new files each time the computer is turned on. I have also seen those that must be run daily. I'd like advice as to the easiest to use, please. Thanks!