In order for a Mac to boot Windows, a standard OS X partitioned disk (called “GPT formatted”) includes a Master Boot Record partition table at the start of the drive. This partition table has only 4 available partition slots (which is the reason for the limitation above), and the total size of all four partitions is limited to 2.2 TB since the Master Boot Record partition is only able to reference data up to 2.2 TB, even if the drive is larger than that. The limitation of 2.2 TB exists on the PC as well, and modern and future PCs will use GPT formatted disks to overcome this size limitation (Windows 7 and Windows 8 provide some support for this partition scheme). Unfortunately, booting Windows with this modern method is not currently supported on the Mac, and not all versions of Windows 7 and Windows 8 support it.

Since it is not possible to get around the 2.2 TB limitation with booting Windows, it is possible to organize the partitions so that Windows is the last of the first four partitons and is within the first 2.2 TBs of space on the drive. Since the Mac can see the remaining space above the 2.2 TB limit, this space can be used for addtional storage space for OS X.