One confusion is why do I have sooooo many partially filled tapes ?   I
don't have this many nodes ?

You might want to do some queries against the volumeusage table.  That data
can be massaged to see what tapes are being used.  You could also use q
content <volume_name> count=1 against all of the slightly used tapes, and
identify which nodes are spread across more tapes than they should be...

Ted