If the tv does have USB3, be sure that your external drive (or enclosure) also does. If it doesn't, then USB2 it is.
Anyways, someone will correct me, if I'm wrong, but I think the bottleneck is the USB connection. Any drive, even the EcoGreen 5400 rpm drives will saturate that.
What you should watch out is that it should be as quiet as possible (and slower, EcoGreen drives are tend to be the quiet ones).