Hi,
It sometimes (often) happens. What's your HD configuration (Fat32, NTSF, Secure Lock Ware enabled?) what's your OS? What's the message in the OS disk managment window (Raw partition or formated but empty partition?)
It could be of different nature, form a local usb controller glitch or a driver glitch, to a corrupted disk header?
Try to turn off the HD (the back switch), then turn off the PC, swap the usb socket (on the hub or your PC). Turn on the PC, then Turn on the HD.
Get back here if you did it already and it did not help.