The setfacl command should be run on the /media/user directory; and, you shouldn’t need the recursive (-R) option. Something like the following should work: sudo setfacl -m u:plex:r /media/pi
However, best practice is to create a static mount point for the disk. Here’s some Information regarding how to set up a static mount to work with Plex:
[Edited to remove inaccurate auto-mount terminology].
