Similar to the Raid idea (but not nearly as effective), I use Allway Sync, which I have monitoring both locations (my external drive, and internal), and you can have it "syncronize" files (remote old files, update to current, and add the new) by:
-When removable device detected
-When file changes are detected (wait X minutes after changes)
-Every ___ Day,week, Etc.
-Before log out
I just use the free version.
What I've got it doing (with some very important files--I'd be dead if I lost 'em...) is:
Syncing with another PC on the network, which is a web server (low-use, of course), and have it clone it once again to an alternate location.
Now there's being paranoid for you...