I think I saw a post earlier that stated the last macOS update undid the Files and Folders access permissions Plex Media Server needs to deal with external Hard Drives.
To check, open the System Preferences on your Macintosh (from the Apple menu in the upper left hand corner). Then open the Security & Privacy panel. Then you should be able to scroll down to the Files and Folders item. Then check that the Plex Media Server has Removable Volumes checked like in this screenshot:
You might have to restart the Plex Media Server application by quitting and restarting for the checkbox to take effect.
Good Luck
Chris
