Since you didn't say what version of Windows you're using, I'll give you the instructions for Windows 7. What you want is the Disk Management function in the Control Panel. In Windows 7 click Start -> Control Panel -> Administrative Tools -> Computer Management -> Disk Management. Now with the external drive plugged in and powered on, look for it in the map of disk devices. You want it partitioned with a drive letter assigned. Once you have a valid partition on it, it will assign a drive letter, and you can then format it.
If you have another version of Windows, the procedure will be slightly different, but not drastically different.
hi, i started formating an usb external hard drive with command prompt,and i think i exited the program before it finished formating.now i can`t open the external devise.can i do anything to fix that