I've discovered a fantastic feature - you can plug in a USB hub to one of the USB ports, and then simply plug in as many USB drives as you want into the hub - each simply sits in an extra folder / layer as seen in the WD HDTV's menu.
Simple, and perhaps obvious, but good to know that this works. Since i now have 2 x 1TB drives under my TV i was worried that these would get full and i'd be stuck for further additions... well not now
I'll also add that in the 6 months now that i've owned it, there have been 2 firmware upgrades (nice to know that WD are still supporting the device
), the best extra feature is that when the device is turned off it stops polling the external drives, hence allowing them to power down too. Nice!