100% a permissions problem. (you see the top of the directory but cannot read the directory)
That didn’t work… Plex dont see any folders in my external hard drive…
Did you follow this procedure?
Common mistakes:
Letting the USB be discovered and automounted to end up in /media/My_username/Disk_name
Manually mounting it via /etc/fstab but FORGETTING to set the permissions of the mount-point directory BEFORE the mount. (you do set permissions twice)