'Course TSM is smart!

We do what you describe every night - saves a lot of tape mounts, especially
since our onsitetape is collocated:

backup stgpool diskpool offsitecopypool
migrate to onsitetape
backup stgpool onsitetape offsitecopypool


BACKUP STGPOOL is always INCREMENTAL - TSM checks the DB and only copies
files that aren't in the destination copy pool already.