To work around this limitation, before you run the Boot Camp Assistant you can manually create a FAT32 partition that is 2TB or less, using the OS X Disk Utility. The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows.” The Solution “The startup disk cannot be partitioned or restored to a single partition.

The end result will be an error message (also seen in the image below) that states: The root cause of the problem is that the Boot Camp Assistant uses the FAT32 format when setting up a Windows disk. Mac owners using disks that exceed 2.2TB may find it difficult to set up a Windows disk using Apple’s Boot Camp assistant.