Understand now. I'd suggest creating a smart playlist using "Create Playlist" -> "Smart Playlist..." tool bar button. Add a a rule into the smart playlist like:
[File path] [starts with] [\\RobinBar\music\]
and click OK. That will create a playlist that contains all the files that are from your back-up drive & that you wish to remove from your Mezzmo library (you can right-click on the playlist and click Properties to edit the rule, etc. if you wish to). Now, select the playlist in the tree in Mezzmo and press DEL key. You will be prompted to remove the playlist and, in addition, you should select the "Also delete the files from your Mezzmo library" checkbox and click Yes. This will delete the playlist and all the files listed in the playlist. Given there are 16K files, it could take a few minutes or so.
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