http://msdn.microsoft.com/en-us/library/ms190190.aspx - Back Up and Restore of System Databases -

The linked document states "You cannot backup the tempdb system database."

:ermm:

I guess you could read that as "why are you wasting your time trying to back up tempdb when by design, everything on it will be permanently discarded when the server instance shuts down?",

:crying:

but in this instance I interpreted it as "You cannot backup the tempdb system database, differential or otherwise."

sigh, -1