If you look into using the DISKPART command from the CMD window I bet you could clean the disk and then you could format it.
From memory it is something like CMD run as Admin--- then you type DISKPART than LIST DISK then SELECT DISK 1 (I assume zero is your Windows drive and the next drive might be your USB HDD but only you know for sure) then you type CLEAN and then you type QUIT. Just make sure you are cleaning the correct drive. That's pretty much all that can go wrong. Once you do that you can use DISK MANAGEMENT to format it.