Try it again after rebooting the computer, if it still fails to show try a different usb port, preferably one of the rear ones, no usb hubs. If it still fails see try another usb cable, if that doesn't work, it could be a faulty hdd box.
If it can be opened without wrecking it, try connecting the drive directly to the motherboard connector, if you have a matching one for your drive.
It depends if the drive is sata or pata connection. Some newer boards don't have any pata connectors.