If you don't have data on it or something you really need to keep, open the Admin command prompt again and use diskpart to clean the drive.
The following are the commands you would use, with an Enter after each line. Just make very sure you are looking at the correct drive before you clean it. One way to make sure is to list the partitions and see if they agree with what you see in disk management.
Diskpart
list disk
The one you want should be Disk 1, but make sure.
list partition
If the partitions agree with the drive you want,
clean
If you want it converted to GPT,
convert gpt
exit
You should now be able to format the drive as necessary. If you convert the Drive to GPT from within Windows, it will put the 128 MB MSR partition at a 17K offset. If you want to install something on the drive, you may need to clean it again, or use Diskpart to remove that partition or during the actual Install process.