Preparing your Fusion Drive Mac for the macOS High Sierra install

Beta versions of macOS High Sierra made a change in the disk format of systems by converting them to use the new Apple File System. The initial release of macOS High Sierra will provide support for the new Apple File System as the default boot filesystem on Mac systems with all-Flash built-in storage. If you installed a beta version of macOS High Sierra, the Fusion Drive in your Mac may have been converted to Apple File System. Because this configuration is not supported in the initial release of macOS High Sierra, we recommend that you follow the steps below to revert back to the previous disk format.


BACK UP YOUR DATA.

These steps include destructive operations that will delete all data contained on the Apple File System Fusion Drive. Please back up your data with Time Machine or another backup solution before proceeding.


Using the macOS High Sierra Installer (requires dedicated external drive of 10GB or greater)

1

Use Time Machine to back up your data.

2

Download the latest High Sierra Install Assistant from Mac App Store.

3

Create a bootable installer on external drive using createInstallMedia.

Full steps at https://support.apple.com/HT201372

sudo /Volumes/InstallAssistant/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/<external drive>

where <external drive> is the name of the drive where you wish to create a bootable installer.

Note: createInstallMedia will erase the selected volume.

4

Choose the macOS High Sierra installer as your startup disk.

Ensure your external drive from step 3 is plugged into your Mac.

Hold down the Option key immediately after turning on or restarting your Mac.

Release the Option key when you see the Startup Manager window.

Select the Install macOS High Sierra boot volume, then click the arrow or press Return.

5

Once the installer has started, select Disk Utility from the list of macOS Utilities.

6

From the View menu choose Show All Devices.

7

Select the container that's part of your internal drives.

8

Click the Erase button. Change the format to be "MacOS Extended (Journaled)". Change the name to something new, such as "CoreStorage Fusion."

9

Quit Disk Utility.app

10

Choose the "Reinstall macOS" option and select the "CoreStorage Fusion" volume as the target.

11

After install completes you'll be brought to Setup Assistant. When asked if you'd like to migrate data, choose to migrate your data from the Time Machine backup you created in step 1.


Using Internet Recovery (Recommended for advanced users only. Requires a Mac running 10.12.5 or later)

1

Use Time Machine to back up your data

2

Start the Fusion Drive-equipped Mac using Internet Recovery

Hold Option + Command + R keys after powering on the Mac. You should see a spinning globe indicating that Internet Recovery has started.

3

On a Mac running 10.12.5 or later, open the Terminal app, which is in the Utilities folder of your Applications folder.

4

Use the diskutil command to determine which internal disks are a part of the Apple File System Fusion Drive.

diskutil list internal should show output like:

$ diskutil list internal
/dev/disk0 (internal):
     #: TYPE NAME SIZE IDENTIFIER
     0: GUID_partition_scheme 121.3 GB disk0
     1: EFI EFI 314.6 MB disk0s1
     2: Apple_APFS Container disk2 120.9 GB disk0s2
/dev/disk1 (internal, physical):
     #: TYPE NAME SIZE IDENTIFIER
     0: GUID_partition_scheme *4.0 TB disk1
     1: EFI EFI 209.7 MB disk1s1
     2: Apple_APFS Container disk2 4.0 TB disk1s2
     3: Apple_Boot Recovery HD 650.0 MB disk1s3
     4: Apple_KernelCoreDump 1.1 GB disk1s4
/dev/disk2 (synthesized):
     #: TYPE NAME SIZE IDENTIFIER
     0: APFS Container Scheme - +4.1 TB disk2
        Physical Stores disk0s2, disk1s2
     1: APFS Volume APFS_Fusion_Volume 280.5 GB disk2s1
     2: APFS Volume Preboot 22.0 MB disk2s2
     3: APFS Volume Recovery 527.9 MB disk2s3
     4: APFS Volume VM 2.1 GB disk2s4

Note: the physical disks that comprise the Apple File System Fusion Drive are disk0 and disk1 in this example

5

Unmount the Apple File System Fusion Drive:

diskutil unmount force disk2s1

OR

diskutil unmount force /Volumes/APFS_Fusion_Volume

6

Erase each of the physical disks identified in step 4

diskutil eraseDisk JHFS+ SSD disk0

diskutil eraseDisk JHFS+ HDD disk1

7

Create a CoreStorage Fusion Group

diskutil cs create "Macintosh HD" disk0 disk1

8

Create a CoreStorage Fusion volume

diskutil cs createVolume "Macintosh HD" JHFS+ "Macintosh HD" 100%

9

Quit the Terminal app.

10

Choose the "Reinstall macOS" option and select the "CoreStorage Fusion" volume as the target

11

After install completes, the Setup Assistant will start. When asked if you'd like to migrate data, choose to migrate your data from the Time Machine backup you created in step 1.