Network Plex directory displays folders on 1st mounted drive folders, not new 2nd mounted drive

First order of business is to make certain the drive is properly mounted on the host.
If it’s not properly mounted there, it won’t work correctly over the LAN.

Need to confirm how the drive is mounted with the USB (the options, mount-point dir, and permisssions).

I’m going to point you to NFS and SMB/CIFS mount how-to’s as well

Give this a read through and see if it points out what you’ve overlooked.