OK, "delete volhist type=dbb tod=today" will not delete all of your data
base backup volumes...
TSM will never delete your latest/only db backup copy.
If you are trying to reuse that same tape volser DB_DO you will never be
able to do it !
You have to have AT LEAST TWO volumes, one which remains the active one
while a new one is created...
then (once a new one on a second volume is created) if you do a "delete
volhist type=dbb tod=today" you can free the first on  to be reused...

hope this helps...
Dwight