In order to regain the space on the Data Domain you must relabel te tape. Marking a tape as recyclable does not free up space. Even after a tape has been relabled, the space will not be freed up until a Data Domain "Clean" has completed. To view tape impact on a data domain, open a PuTTy session to it and enter:

filesys show space.  This will yield a summary showing pre and post usage. for a specific tape use

vtl tape show all

or

vtl tape show barcode {barcoide}