Downgrading an APFS-formatted Fusion volume from Mojave

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If you upgraded your Mac to macOS Mojave and have decided to downgrade for one reason or another, the procedure is usually pretty straightforward. Fusion volumes, however, introduce a complication. Upon upgrading to Mojave, a Fusion volume will be converted from HFS+ to APFS. If you want to downgrade to High Sierra (or any earlier OS), you must reformat that Fusion volume as HFS+. Because APFS Fusion volumes are not handled gracefully by High Sierra, however, the procedure is a bit tedious. The following steps will help you downgrade your Mojave Fusion volume to High Sierra.

Warning: These instructions will permanently delete the contents of the two devices that belong to your Mac's internal Fusion device. If you're uncomfortable with any of the steps in this process, please don't hesitate to ask us for help.

  1. Boot from your CCC bootable backup that you intend to restore from (e.g. macOS High Sierra or earlier).
  2. Choose "About this Mac" from the Apple menu to verify that your Mac is booted from your backup volume.
  3. Open Disk Utility.
  4. Choose "Show all devices" from the View menu.
  5. Identify the two devices that belong to the APFS Fusion volume. Typically one will be an SSD and the other will be an HDD, and both should be in the "Internal Devices" section of Disk Utility's sidebar.
  6. Erase the SSD Fusion member as "Mac OS Extended, Journaled". Name it "FusionSSD" so it's easy to identify later.
  7. Erase the HDD Fusion member as "Mac OS Extended, Journaled". Name it "FusionHDD" so it's easy to identify later.
  8. Quit out of Disk Utility.
  9. Open Carbon Copy Cloner.
  10. Click on the "FusionHDD" disk in CCC's sidebar.
  11. Click the "Recovery HD..." button at the bottom of the window.
  12. Click the "Create Recovery HD" button. If that button is disabled, don't worry – this step isn't essential.
  13. Quit out of CCC
  14. Open the Terminal application, type the following command, then press the Return key:
    diskutil list
  15. In the list of devices and volumes, find and make a note of the device identifier (in the IDENTIFIER column) associated with FusionSSD and FusionHDD. For FusionSSD, we will use the whole device identifier, e.g. disk1, whereas for the FusionHDD, we will use the volume device identifier, e.g. disk5s2.
  16. Type the following command in the Terminal, substituting the device identifiers noted in the previous step, then press the Return key:
    diskutil cs create "Macintosh HD" SSD_Whole_Device_Identifier HDD_volume_identifier
  17. The previous command will create an empty Fusion device, and print out a "Logical Volume Group" identifier. Select that identifier and copy it to the clipboard.
  18. Type the following command in the Terminal, substituting the logical volume group identifier noted in the previous step, then press the Return key:
    diskutil cs createVolume Logical_Volume_Group JHFS+ "Macintosh HD" 100%
  19. Quit out of the Terminal application.
  20. Open Carbon Copy Cloner.
  21. Create and run a new task, specifying your backup disk as the source and the new "Macintosh HD" Fusion volume as the destination.
  22. When the restore task is complete, open the Startup Disk Preference Pane in the System Preferences application. Reset the startup disk to Macintosh HD, then reboot.