Hi there,
Well it depends how are you trying to create that restore job. By default BE will try to restore it to it's original location (exchange disk in this case), but you can also choose to restore it to alrernate location.
I think that you are trying to restore it on it's original location (exchange) so probably that is the drive that error is talking about.
You should try to check your exchange disk where the DB is located, and see if there is sufficient disk space in it. But I don't see why would you have such a problem if there is 10GB of free space on your exchange server (since i doubt that you have to restore inbox larger than 10GB :))
Hope that I understood your question correctly, and that I helped.
regards
Ivan