That is a very important information that you are not trying to access local drives but a network file share (i.e. a NAS). It is missing in your first post above.
Are you running Plex server regularly (i.e. after logging into your Windows user account)?
Not per task scheduler, or as Windows system service?
Nor per “Run as Administrator”?
Things you need to know:
Drive letters which have been connected to network file shares are not available to Windows services.
They are also only available to the Windows user account who created them.
Plex can only “see” drive letters which were already present when Plex server was started. If you connected those drive letters after Plex server was started, then you need to stop and restart Plex server.
You might wanna look at this: How to add files on a NAS to a Plex library on Windows
