Plex not seeing folders on external drive

[ChuckPa]

Give a read here.

Linux Tips

Tips & Tricks of the Linux trade. Below, you’ll find a collection of tips, tricks, and how-to’s to help make setting up, administrating, and using Plex Media Server on Linux a little easier. Reporting problems Using other hard drives (Windows or Linux) with PMS Customizing your Plex configuration on systemd based systems Using other EXT4, NTFS, or other drives (internal or USB) Mounting CIFS + SMB network shares for Plex use Mounting NFS network shares for Plex use Movin…

4th one down will probably be of most help.

It shows how to mount the drives.
Where you mount them is your choice as long as there are no conflicts