I had the same problem with reformatting using the WD instructions as pointed out by Hamlet. I am using a My Passport Ultra with a Macbook running Mac OS X 10.11.15 (El Capitan).
I got around the issue by using the Erase command instead of Partition. This allowed me to do exactly what the Partition command does, erasing anything on the drive (there was previously nothing on the drive and was brand new), formatting in MS-DOS(FAT), and using the Scheme: GUID Partition Map.
