Important Note: This help document is for reference only, superceded by updates to OS X that include support for 3TB drives and Fusion drive in Boot Camp Assistant.  Use Boot Camp Assistant to create the Boot Camp partition.



Installing Windows on a Boot Camp partition on a 3 TB Fusion Drive


Does Boot Camp work on the 3 TB Fusion Drive? When first introduced on the 2012 iMacs, Apple didn't officially support running Boot Camp on the 3 TB Fusion drive (and still don't as of this writing). Until Boot Camp is officially supported, we are providing a tutorial on a way to get a working Boot Camp partition on a 3 TB Fusion drives without erasing the Mac partition or needing to migrate data. When finished with this tutorial, you will have three partitions: OS X partition with all data intact, a 1 TB partition containing Boot Camp and a 1 TB partition with additional storage space for OS X.

Boot Camp Assistant

When launching Boot Camp Assistant on a 3 TB Fusion Drive iMac, a message will appear that shows that the Boot Camp Assistant does not support installing Windows on this Mac. In order to install Windows on a 3 TB Fusion Drive iMac, you need to create 3 partitions. Due to the limitations of how the Mac boots Windows, the Windows partition must be contained within the first 2.2 TB of drive space.

Boot Camp Assistant Screen Shot

Standard Configuration

When you first unbox a 3 TB Fusion iMac, the iMac has a single volume with 3.12 TB available. This is the 3 TB standard hard drive and a 128 GB SSD. This volume uses Core Storage, and you can see that the type is designated as "Logical Volume Group". Disk Utility will only allow you to create one other partition, and since the Windows partition must be situated after the Mac partition. Disk Utility does not allow you to create the Windows partition after the Mac partition, as that would extend beyond the 2.2 TB boundary. To accomplish this, create a 1 TB Mac partition, and a 2 TB Extra data partition. This extra partition will be outside of Core storage, so we can split it into 2 parts and both the new parts will be outside of Core Storage. When finished, you'll have 3 partitions: Mac (JHFS+), Windows (FAT32), and Mac (JHFS+) in precisely that order.

Open Disk Utility in the Utilities folder, and then:

1. Select the top level Macintosh HD
2. Select the partition tab.

Disk Utility Screen Shot

Create the Extra Partition

In order to end up with three 1 TB partitions, you need to split the 3 TB partition into a 1 TB Mac (HFS+) partition and a 2 TB partition. We will then split the 2 TB partition again into two 1 TB partitions. This is required because Disk Utility only allows you to create one additional partition. The terminal command "diskutil" can then be used to split the 2 TB partition into two 1 TB partitions.

To split the 3 TB Core Storage partition into a 1 TB Core Storage partition and a 2 TB non-core storage partition:

  1. click the "+" button
  2. drag the slider between the two partitions so that the top partition is a bit less than 1 TB and the bottom partition is about 2 TB.
  3. Make sure that the name of the second partition is "Extra" since that name will be used in the next step.
  4. Click Apply

This will not erase the Macintosh HD, but will just resize it to make room for the new partition.

Disk Utility Showing Partitioning Screen Shot

Split the Second Partition

CAREFUL: When splitting a partition, the partition that is going to be split will be erased. Do not run these commands unless you have a good backup and are sure that the partition you are splitting does not have any data you need to keep.

Since Disk Utility cannot create more than one additional partition when one of the partitions is managed by Core Storage, you must use the diskutil command. To do this, open Terminal in the Utilities folder. The "diskutil" command has an option called "splitPartition" that will create two partitions from one partition. This will erase data on the partition you are splitting, but since we just created this new partition in the prior step, it should not contain any data.

Let's take a look at the various parts of the command.

sudo diskutil splitPartition /Volumes/Extra 2 MS-DOS WIN 1T JHFS+ MacBonus 0b

1. sudo diskutil splitPartition: diskutil is the utility to modify disks, and the splitPartition verb tells it what we are doing. The "sudo" portion runs the command with administrator privileges.
2. /Volumes/Extra: The partition that is going to be split
3. 2: The number of partitions to split the original partition into.
4. MS-DOS: the format of the first partition. This will be the bootcamp partition
5. WIN: The name of the boot camp partition that will be the first part of the old partition.
6. 1T: The size of the new bootcamp partition. It will be 1 Terabyte.
7. JHFS+: The format of the second partition created from the split.
8. MacBonus: The name of the second partition created from the split.
9. 0b: The size of the second partition created from the split. Since the last partition of a split always fills up the remaining space, we can specify 0 bytes and let diskutil figure out the exact size.

Type the following command in terminal, and press return:
sudo diskutil splitPartition /Volumes/Extra 2 MS-DOS WIN 1T JHFS+ MacBonus 0b

If you are prompted for your administrator credentials, enter them when prompted.

Results after Splitting

You should now have 3 partitions, each about 1 TB each. The first partition, Macintosh HD, still is controlled by Core Storage and will have all the advantages of a Fusion drive. The second partition, WIN, will allow you to install Windows or restore a Winclone image on to it. The final partition, MacBonus, has the remaining space. The Windows volume must be within the first 2.2 TB of hard drive space.

Finder Window and Disk Utility showing new partitions

Restart to Proceed to Install or Restore

After repartitioning the disk, you should restart since it will assign the expected partition numbers after restart. If you are installing Windows, you can0 now install Windows 7 or Windows 8 on the new Windows partition. Since the 3 TB iMac does not have an internal optical drive, you either need to purchase the external optical drive, or create a bootable USB stick from a different Mac that allows you to run Boot Camp assistant. If you are restoring a Winclone image, restart and follow the next steps.

Apple Menu Showing Restart Option


After restart, open System Information in the Utilities folder and verify that the Windows partition is disk0s4.

System Information screen shot showing partitions


Since the Windows partition has been formatted as FAT32 (MS-DOS), you can restore a Winclone image on it.

Winclone screen shot showing Restore Volume


With the introduction of the 3 TB Fusion drive on the iMac, Apple has created an excellent balance between the speed of SSD and the mass storage of standard hard drives. However, it does cause some issues with installing Windows on your Mac, and you need to make sure that the Windows partition is within the first 2.2 TB of the hard drive and is on partition 4. Now you can set up your Fusion drive to take full advantage of the speed of the Fusion drive, as well as a large amount of storage space.