I added a new drive to my NAS and moved files.
How to get rid of the unavailable duplicates?
Plex for Windows—General Version 1.27.2.1599-7689068b
DSM 6.2.3-25426 Update 3
Plex in DSM 1.21.3.4021-5a0a3e4b2
Trash empty
Scanner/Agent: Plex Movie
Manage Library>Refresh all metadata done
Just a personal opinion, but I’m not sure that “Empty trash automatically after every scan” is always good advice. With that enabled, even a temporary communication failure with a drive will result in the trash being emptied automatically (especially if you have the server configured to automatically detect changes in the library.) When the drive is reconnected, the files will all be added as new items in the library. I’m pretty sure I’m right, but maybe @trumpy81 can comment.
As a side note, the order of the steps for the Plex Dance given here will only work if you have “Empty trash automatically after every scan” enabled. Steps 4 and 5 have to be switched, otherwise.