Not an expert, but when has that stopped me from having an opinion? Using an external hard drive for backups isn't any better than having a separate internal hard drive, with this exception: if you have an external backup you can store it in a separate location. That way a burglar or a fire/flood/earthquake/whatever can't take your primary and backup data all at the same time. The problem with that approach for us regular folks is, how often do you really think we are going to go dig the ehd out of it's secure storage area and make a backup, then take it back to the secure storage area? Someone who is really serious about this and has a lot of irreplaceable files really should look at automatic backup software and internet storage. I personally don't have need for all that, but I do run a separate internal drive for weekly automated backups (same backup is also on a separate partition on the primary hard drive for redundancy). And the Ghost image idea Sean mentioned is a real lifesaver. I've used it on a couple of occasions to fix a totally screwed software installation (Norton has developed a bad habit of selling software that is incompatible with the earlier versions of the same software, and the uninstallers don't fix the mess). It would be a real lifesaver in the case of a total hard drive crash as well, saving untold hours installing and customizing the operating system and programs on a new drive.
Jerry