The libraries are empty on Linux Mint last edition

[pshanew]

You’ll likely need to use the information at the link below to create manual mounts for the external/NTFS-formatted disks:

Linux Tips

Using other hard drives (Windows or Linux) with PMS This How-To is written to help with Internal and External drives. It also is written to demonstrate how to cope with NTFS and EXT4 formatted drives. By logical extension, any format supported by Linux can be handled using the technique shown here. In all cases, the goal is to give user plex permission to read your media files which Linux doesn’t allow by default due to its strict security model. We will walk through the steps needed …

The auto-mounts which Gnome/Nautilus create cannot be accessed by Plex no matter how you set their file/directory permissions.