Why can't I backup a My Book Live to a My Book USB Drive on a Mac?

I have a My Book Live Network Drive. Now I’m trying to backup the network drive to a USB drive on a Mac mini. I have been to the apple store. They had a real fast answer and said it can’t be done using time machine. That is as far as they will talk about. So has anybody able to backup a network drive to a USB drive using a Mac?

Check the link below for the list of available options that you have. You can also check page85 of the manual.