USB Connection

I have a dual slot external HD dock with a 4TB drive in each slot.  I works perfectly on my PC (Win 7).  When I connect it to the WD TV Media Player it can only see one of the drives via the USB port.  I tried it with a 4TB drive an an old 500MB dive and it could see them both.  Am I exceeding some kind of limitation by using two 4TB drives?

Have you tried using just one 4TB drive at a time to confirm they both work? Don’t have any other drive plugged in at this point. What’s the file format of each 4TB?

Yes they will both mount properly if used one at a time.  If I use the dock (with both drives) on any of my three PC’s they both mount perfectly.  I am very confident the external equipment is working properly.  However on my PC’s I connect via USB 3 and of course the media player is a usb 2.  The dual doc is backward compatible with usb 1 and 2.