Open an elevated command prompt (right click run as admin), then type:
diskpart
list disk (if the external shows up note the number of the drive that it has)
select drive n (where n is the external drive)
clean
create partition primary
format FS=NTFS label=xxxxxx quick (or whatever letter you want it to have that does not conflict with your other drive letters, and name it where the xxx are, like storage drive)
exit