Just in case anyone's not aware of it, Microsoft "Synctoy" (a free download) is an excellent utility for keeping data backed up on an external drive. You designate the folders you want to back up, and when it's run, it syncs the contents of the external drive with the internal. You can configure it either so that it maintains the external data as a true copy of the internal (matching new files, changes, and deletions), or such that it never does deletions on the external copy.
More details at:
http://www.microsoft.com/prophoto/do...nctoybeta.aspx