Hi Dean,
I believe I've found an effective workaround: when setting up your backup job, click "Add a network location..." when choosing a location to save to. From there you can seemingly specify any subfolder structure you like, so you ought to be able to backup to any locally attached drive's administrative share, too.
For example, if you wanted to backup to "D:\backups\", you could specify a network location of "\\[your machine name]\D$\backups\", excluding the quotes of course. Just enter the username and password of an account with permissions to that folder and away you go.
This should work as long as you haven't removed the administrative share for whichever drive you're backing up to.
Shiro