Hi Bob,
This is new to me, I had thought that it would work with both a File Library Device and a Backup to Disk Device, which is usually reserved for a StoreOnce device
That's not necessary. File Libraries have always automatically purged all meanwhile expired media on every write access. Given that was a quick removal of some large files (the only thing that could slow it down being a file system that doesn't like this specific operation, like ext3, and maybe DFMF overhead in case the option was in use), it didn't matter that happened before it actually started to write. The method has changed with B2D/StoreOnce for good reasons (imagine the havoc it would wreak when media was purged and thus housekeeping started exactly at the begin of every write activity like a backup). Could also happen with a File Library on a StoreOnce Share, but at least you have housekeeping windows there.
HTH,
Andre.