Unpowered EHDs can be connected to a port or a hub on any of the 3 built-in ports. I would not try having more than 3 active EHDs. The 3rd can only be used for transfers in or out as there are only 2 (Externals 1 and 2) in the menu for playback next to the DVR red button.
The 3 can all be run at the same time and hub or port does not seem to matter. That assumes hub is not speed limited like a keyboard only USB1.0 is. Then again I doubt USB3.0 is a help over USB2.0, so far. Buy USB3.0 if the same price IMO.
-Ken
Question above: have you had more than 3 transfers at once? Guess I should try. -K