Hey there, @quinncollard!
Are you sure the external HDD is properly recognized by your system? I'd advise you to check how the backup utility will react to changing the USB cable connecting the external and also plugging it to a different USB port on your computer.
If you have any important files currently stored onto it, I'd advise you to back them up on your computer and then try
reformatting the HDD through Disk Management (Just make sure you select the correct Disk # for your external, so that you don't suffer any data loss or accidentally re-format one of your internal drives).
Give these a try as well and let us know how it all goes.
Cheers,
SuperSoph_WD