Mitel just published (12/19/11) an article on the KB regarding problems backing up a 3300 using windows 7.
Apparently the problem is that by default the root (c:\) cannot be written to.
The solution is to write to c:\backup\
I've had issues backing up to network drives so I don't bother with trying any more.
I've always backed up to c:\backups\ so I never knew there was a problem.
Just something to be aware of.
Ralph