Filesystems which are automatically mounted under /media/username (by udisks2) have ACLs applied to them; these trump any perms you set with chmod.
It’s recommended to create static mounts for your disks using the information in this how-to:
You could also try the following. This will modify the ACLs associated with /media/mypi/ to include user plex with read/execute permissions.
sudo setfacl -m u:plex:rx /media/mypi/
