I don't know why the comment below this "What will you do if your house burns down?" is marked as dead ... it's a very valid question.

Backing up onsite is almost as bad as no backup at all. You are protecting against hardware failure ONLY.

If your house burns down you are screwed. Also, if someone breaks in and steals your computer, do you think they aren't taking your external drives as well?

Offsite backup is mandatory if you really care about not losing your data.