I know it sounds silly too, but one thing lots of people do, is delete the stuff they've backed up from their machine. The idea of backup, is so that if your machine goes tits up (so to speak), then you've got an alternative.
If you backup and then delete something, you've got just as much chance of your external HD packing up, as you have your machine.
Don't be afraid, with raid.
We use these things called cds at work.
