That very much looks like a permission issue… default mounting assigns the drive either to root or the active user which are both not visible to the plex user used by the Plex Media Server.
Check out these tips & tricks records (depending on the protocol used to mount the drive): Mounting Samba/CIFS / NFS network shares for Plex Media Server.
Also… if you’re having issues with the connection of a drive, disable automatic deletion of your media from the library by unchecking Settings > [Server] > Library > Empty trash automatically after every scan. This way, if a drive’s connection is lost, Plex will keep the record but display a trash can on the poster… to actually delete it from Plex you’ll need to manually Empty Trash for that library using the library context menu.
