I have installed Plex on a Raspberry Pi 3B which is working except it can’t see media folders on a NTFS external disk.
The folders and contents are available in the Raspberry Pi File Manager but not when I try to add a library from within Plex.
The folders have been set to read/write in Windows.
If the media is mounted (visable) as being under /media, only you will be able to see the contents. Plex will see the top only.
Only after you follow the How-To, moving the mount-point to a new location, and setting permissions correctly, will PMS be able to see the media as well.