I can't tell you why, but I have found none of these surround sound systems, or dvd recorders will play from usb hdd's. They will play from usb sticks fine.
I have a JVC surround sound system and a LG dvd recorder fitted with a USB port and neither will play from a hdd.
I have had queries from others on the forums with the same problem.
The LG manual states; Connect a USB Memory Stick.