How to stop Plex from removing titles from library that are no longer on disk

[OttoKerner]

Put a placeholder file there, the size of a few bytes.

Is there any possibility for browsing my DVD collection in my media server?

Plex can be fooled: Take a text editor. type for instance this into it: Manchester by the Sea (2016) save the file and then rename it to Manchester by the Sea (2016).mkv put this file into your ‘Disc library’ folder and let Plex update the library. Done