Plex Media Server can't find files on USB drive connected to Raspberry Pi 3

@“henk.visser@yahoo.co.uk” said:
It sees the mount point “/media/pi/FREE_AGENT” no shows it as empty

You missed the one and only important document. The following guide will help you to fix your problems.

http://forums.plex.tv/discussion/279063/using-ext-or-ntfs-drives-internal-or-external-on-linux