Plex won't load library - raspberry pi 4

[tom80H]

sounds like a permission issue… you need to make sure plex (user) has access to those folders and the drive is properly mounted.
you might want to check the related post in the tips & tricks section:

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 …