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Workaround for booting Windows on Macs with 3 TB drives

By John Rizzo

Mac uses with 3 terabyte drives have experienced frustration with not being able to install Windows via Boot Camp. OS X Mountain Lion v10.8.3 and later is supposed to fix this, but often it doesn't work. There is workaround that involves a special way of creating the Windows partition, according to Twocanoes (a software company that makes WinClone, software for making copies of Boot Camp partitions).

Boot Camp Assistant won't partition 3 TB hard drives or Apple's hybrid hard-disk-drive/solid-state-drive Fusion drives, available on Mac minis and iMacs (2012 and later). The problem with these drives is that they are larger than 2.2 terabytes. In order for Windows to work with Boot Camp, the Windows partition must fit entirely within the first 2.2 terabytes of the drive, and must also come after the OS X boot partition.

The workaround that Twocanoes suggests is break the OS X partition into two one containing OS X itself, and another containing the Mac's data. This enables the OS X partition and the Windows partition to fit within the first 2.2 terabytes, with the second Mac partition coming after that.

This screen shot of Disk Utility gives the general idea.

Twocanoes recommends using Disk Utility to create a partition for Windows instead of the usual method of using Boot Camp Assistant. That's because Boot Camp Assistant puts the Windows partition at the end of the drive, after the first 2.2 terabytes, and there's no way to change this. A downside to partitioning with Disk Utility is that it will erase the entire drive, so you'll have to back up first.

To complicate things slightly, Macs actually have two additional partitions that are not shown in the screen shot because they are very small. The first is called the EFI partition, which sits in the first position before the OS X partition, which must sit in the second slot. In third place is a small Recovery partition (on Macs with OS X 10.7 Lion and later). The Windows partition is actually the forth partition, and must be so. All this must be within the first 2.2 terabytes. The Mac data partition comes after that.

The Twocanoes page for this workaround describes how to create and order the partitions and how to verify that the partitions are in the right place.

It's worth noting that while you can use WinClone to migrate Windows from a Boot Camp installation on another Mac, this workaround does not require the use of WinClone or any of Twocanoe's products. Hats of to them for providing this fix.