NW will NEVER overwrite any media just because it is recyclable. Why? - Simply 
it is the policy to protect (accidental) data loss as long as possible.

I have to admit that this behavior is not beneficial for a disk media as all 
the save set should have been deleted in the meantime. I just believe that NW 
uses the same policy as for tape media.

Something i forgot to mention: To the best of my knowledge for disk media, you 
must activate AMM that the relabel process will be triggered automatically. But 
once again ...

NW will only recycle it when it has expired but
- when all its save sets have entered the state recyclable (have been deleted) 
AND
- when he needs a media (from that pool, depending on your pool settings)

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