1. You've indicated that the USB external HDD is listed in the "Disk drives" section of Device Manager. (At least I presume you're referring to DM, right?)
2. So the USBEHD is not shown in Disk Management?
3. Has that USBEHD worked OK in the past or is this the first time you're using it?
4. Is the USBEHD a commercial unit or is it a HDD which you installed in a USB enclosure?
5. Are you able to remove the HDD from its enclosure and install the disk internally in your system?