Hard drives will always fail at some point, that's just a fact.
The key thing that most people don't seem to grasp is that if you are using the external drive as a backup... the files MUST EXIST ELSEWHERE. So you can't delete files from your primary drive in your computer just because they are on the backup drive, unless you have a second backup drive backing up that one.
What I've started doing is backing up all my machines across the network to one big internal drive on my Linux box, and then I have an automatic backup of the internal drive to a big external drive.
That way most of my data is in 3 places, so even in the event of a failure, I'm covered.
I'm considering augmenting that with online backup...
[edit] ... this would make a good article series. I think I'm going to start working on that...