Hi @bcolyer 

Let me start by explaining what this operation does.

Erase Spare Media operation ensures that old data from tape libraries cannot be recovered once the media is recycled and overwritten.  This is done by writing EOD (End of Data) at the beginning of the tape (in the first byte) and the DSS (Data Set Separator) pattern is written to the end of the tape to overwrite the existing data.  

You can run the Erase Media operation only on spare media

Please check out our documentation for further information 

If that answers your question, feel free to mark this post as the correct answer.

Thank you