I also have an external drive that I use to back up and I use a service called
Syncplicity. It automatically back up online, will also sync with Google Docs, Zoho and some other services just can't remember which. Cool thing about the syncplicity service is you can run in on different computers, or even provide access to specific files (think clients) and ensure that everyone has the correct version....
Runs in the background and simple to set up. Defininately gets my recommendations.