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Can Mac Fusion Drive Dual Boot Camp

We just got our iMac with the 3 TB Fusion drive in it to do some testing to get Windows installed on a Boot Camp partition. I had a 1 TB Fusion drive and suspected that I could get it working, but wanted to wait until it arrived before posting. There are 2 major issues with getting Boot Camp running on a 3 TB Fusion Drive:

1. Since the EFI uses the MBR partition table to flag the bootable partition (and other things), the Windows partition must be completely within the first 2.2 TB of the drive.

Mar 13, 2014  Splitting the Fusion Drive - A geek's tale. Drive when they buy a 2012 iMac or Mac mini. A Fusion drive is actually two physical drives within the machine. A Boot Camp partition on the SSD. With Boot Camp, you can choose to start your Mac in either OS X or Windows. Unlike the virtualisation apps just mentioned, which run both operating systems at once and thus affect performance, Boot Camp really makes the most of your hardware. Note, though, that you'll need your own copy of Windows to install.

2. The Fusion drive uses Core Storage to combine the speed goodness of SSD with the capacity of a standard hard drive. Windows has no knowledge of Core Storage, and is not able to boot from a Core Storage volume.

Additionally, Disk Utility limits the number of partitions you can create. If there is a Core Storage volume, Disk Utility will only allow you to create one additional partition. The additional partition is not a core storage volume, but rather a standard partition that can be used to boot windows.

The introduction of the 3 TB Fusion drive adds the final wrinkle. Since Disk Utility only allows you to create one additional partition, and you have to put the Windows partition AFTER the Mac partition, you cannot make the Windows partition completely within the first 2.2 TB.

To get around this, you can use Disk Utility to create one additional partition, then use the command line “diskutil” command to split this new partition. It is possible to completely break down the Core Storage volumes and rebuild them, but that requires you to pave and restore the data, which is unacceptable to me. Using the technique outlined in this article, you can create a Boot Camp partition without any loss of data or migration requirements.

Here is the goodness step by step:

Open Disk Utility in the Utilities folder, and then:

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

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.

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.

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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.

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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.

Verification

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

Restart to Proceed to Install or Restore

After repartitioning the disk, you should restart since it will assign the expected partition numbers after restart.

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Installing Windows

If you are installing Windows, you can insert 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. You can also restore a Winclone image.

When Apple announced the Fusion Drive, I assumed it was just a hybrid SSD drive. However, after consulting the internet tubes, I discovered a consensus that no one really knew what the heck it was. We had to wait.

Then the reviewers got hold of it, and lo and behold, it turns out to be separate SSD and standard drives combined with Core Storage. Neat.

The question remained about how Boot Camp is supported on the Fusion drive. Apple’s support document said that you can create two partitions AND THAT IS ALL YOU GET, SO GOOD DAY SIR.

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At the Apple store I had to special order a Mac Mini with the Fusion Drive. Once it arrived, I looked at the Fusion Drive in Disk Utility and it displayed as a single drive and partition:

The terminal shot of the core storage view above shows the different drives. The technology behind core storage has been well-covered in the blogosphere, but a question remained: how does Disk Utility handle repartitioning the drive and how does it appear in Windows?

So I created a new partition in Disk Utility, and sure enough Disk Utility only allowed creation of one additional partition, which could be either HFS+ or FAT32/exFAT. No third partition can be created.

I then used Winclone to restore a Windows image to the FAT32 partition I created in Disk Utility, booted it up and it worked great! From the Disk Management utility in Windows 7, the disk looks like this:

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As expected, Windows sees it as two separate drives. Back in OS X, I checked out the GUID partition table:

Some interesting things here. As you would expect, there is a Core Storage partition on both the SSD drive and the standard drive. Both have a GUID partition scheme and an EFI partition, as expected. However, the SSD has its own Apple_Boot partition. It could be that the Apple_Boot is the OS X System installed on the SSD, but it looked more like the partition to boot into a File Vault 2 volume. A closer look at the mounted partition showed only the basic system files needed by core storage. Similar, if not identical, to File Vault 2.

The Windows partition and a recovery partition exist entirely on the standard drive. As expected, since OS X requires Boot Camp to be last among the first 4 partitions on the drive, the Windows volume is partition 4.

Note that this drive is 1 TB in size. As of this writing, Boot Camp only supports drives up to 2.2 TB in size. Next I’ll be posting an article about how to get Windows installed on a 3 TB or larger drive. This will apply to both Fusion and standard hard drives.