By default, because of the ACLs associated with it, only your user (pi) can access files under /media/pi. This is due to the way the disk is automatically mounted. The recommended way to use extra disks with Plex Media Server is to create static mount points and the necessary entries to mount them there in /etc/fstab. That method is described in @ChuckPa’s excellent Linux Tips post:
Aside from that, if you’d like something quick and dirty to try, you can modify the ACLs associated with /media/pi using the setfacl program. Something like the below should work:
sudo setfacl -m u:plex:rx /media/pi
This will give user plex (the default user for PMS) read and execute permissions to /media/pi. Assuming file/directory permissions are correct for the disk mounted under that directory, Plex should now be able to read the disk’s contents.
