Since you use 'move drmedia' TSM should know that the tapes aren't full but they can't be written to anymore and so will start the expiration process on the data on the tapes.
Are your tapes below the reclamation threshold? (select volume_name, pct_reclaim, stgpool_name from volumes where pct_reclaim => (reclaim_threshold) order by 3,2 asc)
Also, I thought that when you run a 'move data' on a copy tape, it would use the primary copy and create a new copypool tape just like a reclamation process does but I could be wrong (it's happened alot lately)
-Aaron