No idea. Waffle incoming.
The first thought which comes to mind is whether your hard disk is mechanical or flash based. If mechanical, then I would wonder whether the USB port of the TV can provide enough power to it; if it can't then it can't be read. This assumes it doesn't have an external power source.
Second thought is whether the USB port on TV is actually working. Try a USB stick and a small file, like a picture, to see if the port works.
Third thought is whether the TV requires storage to be formatted to a particular system in order to read it. Check your TV manual for information about support.
That's all I've got. I'm sure others will have actual advice.