Microsoft Windows has a disk management feature built in. This feature can be accessed by right clicking on My Computer and selecting Manage. After selecting manage, select disk management in the left pane. Your computer will try to retrieve the hard drive information (Will appear in the Right Pane). If your external drive is recognized, you can reformat the drive from this location. If the drive is shown and there is no drive letter, right click on the drive and select change drive letter. Click on Add and select a drive letter. This often happens if the USB ports are being used with flash drives or you have several mapped network drives.
If the drive is not recognized, you should try it on another computer, try different cables and turn the drive on and off. If the drive is still not recognized, you may want to remove the drive and try to attach the drive to your motherboard and see if the computer can see the drive.
Another solution is to purchase a universal hard drive adapter. These adapters allow you to retrieve files from computers that have crashed or to attach drives to be formatted or tested when repairing computers. (USB Adapter Link).
Third party utilities can detect and force hard drive formats. Dug IDE and Wipeout are two utilities that can be used to low level format (or force format) hard drives. These utilities will completely wipe out and write zeros to the hard drives.