By default, when the Gnome desktop’s file manager Nautilus finds unmounted devices (like thumb drives) it mounts auto mounts them for you in /media/your_username and gives you exclusive access
For you:
Get rid of those other sharing packages you installed. They don’t do what’s needed . They only cloud what needs to be done.
You need to move the drives from where the automounter has placed them AND grant permission so Plex (user plex:plex) can see them.
