I’ve had the same problem before, but I can’t remember exactly how I resolved the issues.
I think you have to have the usb drive running before you boot up your PC or you have to set up the usb drive (plug the usb connector and power supply) after the PC has booted up.
Is the usb the correct version (1 or 2). I tried using a usb 2.0 device with usb 1.1 software (operating system) and it was not able to read the MBR (master boot record) to determine how the hard drive was formated.
Just a few suggestions
There might be incompatibility issues in your hard disk drive to your USB port of your computer system. Remember that there are several types of USB ports like 1.0, 2.0 and 3.0. Make sure that your hard disk drive USB connector are compatible with your computer port.