Hi there
I'm not sure why PLEX needs full write access on your media libraries to update its database.

Only granting READ access I notice that a library update doesn't include the new media files I've added.
READ/WRITE access -- no problems.

I can't see though that PLEX actually does any WRITES into the media libraries - It stores stuff in its own database. You do need a transcode directory with write access if you are transcoding of course.

I know quite a few here use PLEX so just warning - I only allow WRITE access to specific directories / folders on my system and then to specific users and / or processes.