In order to use Plex you need a PMS or Plex Media Server running in order for plex clients to connect to it.
It sounds like your PMS runs on your Macbook and your data is stored on the USB which is a network share which still means in order to see this in Plex clients you need to have your Macbook running.
You might be able to setup some sort of DLNA or UPNP on your EE Brightbox and your if your EE TV supports this as well it might just see those video files outside of plex and play them that way.
Sorry I am not familiar enough with EE so maybe someone with direct experience may chime in.