As a Network Admin/Engineer, I ALWAYS tell people "Back up your back ups!" if it's important! I have 3 External Drives. One I save EVERYTHING to ( I never save anything to my PC's Hard drive for more than about 72 hours, max!) , one I use to back up the first one every 3 months, and the third I use to back up everything at the begining of the year! I also have all my dios in PDF format on a 16 Gig Thumbdrive and on my shared drive at work!

External hard drives are the way to go, but remember, THE ARE HARD DRIVES, TOO! and they are just as prone (if not more so!) to failure. So, like I say, back up your backups! Backup is important, but Redundancy saves the day!