Hi Gordon,
Welcome to Tom's Hardware.
Tell us about your computer, Model #, Manufacturer, MB, Video card, HDD's attached, etc., and OS system and version you are using.
Then with the external USB drive attached, reboot and check in the BIOS in the standard CMOS features, to see if it is recognized there along with your other HDDs, then in the Device Manager to see in the OS recognizes it, and finally in Disk Management to see what it says in the Drive Status area, and Volume Status area.
Then we will have information to help out.