Last week Anne Mini of the awesome blog Author! Author! had a great post on backing up your work. I love a lot of this post. It's more complete and entertaining than my posts on the subject--I even got caught by her comments about your backup sitting next to your computer. But there is one thing I think needs to be added to the great information in Anne's post, and it's regarding her comment that "I literally never leave my house without either my laptop or a full back-up of it on my portable hard drive."
Carrying your backup around with you, whether it be an external hard drive or a laptop or a CD, can be very comforting, but it does come with a certain amount of risk. It's always possible that you could lose your backup or that it could be stolen.
Now, odds are whoever has your backup is not going to be all that interested in your writing. (If you're like me, you also store all your financial backups and your passwords file on your backup thumbdrive, which your average computer-savvy thief will fine much more interesting.) But you never know when the thief (or bored teen who finds your lost backup) might decide to post it on the internet just for grins.
The answer? Encrypt your backups.
I use TrueCrypt on my netbook and thumbdrives, but there are other good encryption tools out there, so do your homework. It's a whole other level of piece of mind.