Navigate to that external hard drive and partition it according to your needs. Under the External heading on the left hand side, you should have one hard drive instead of 3 (I have already partitioned mine). Yours will look a little bit different than mine. On the left hand side, navigate to the tab that says EXTERNAL. To partition the drive, go to Spotlight at the top of your MacBook Screen (Notification Bar) and type in Disk Utility.
If you have an even larger drive, you can create even more partitions for other operating systems like Linux, etc.
