High Sierra: APFS “not supported” on Fusion Drives

In an undated advisory note to those who took part in the High Sierra beta-testing programme, Apple has warned that it will not support the use of APFS on Fusion Drives in the initial release version of High Sierra. That note recommends that anyone who converted their Fusion Drive to APFS during the the beta programme should format that drive in HFS+ before they install the release version of High Sierra.

That note also reveals that:

  • Apple intends that a future version of High Sierra will support APFS on Fusion Drives. It makes no comment about support, present or future, for conventional spinning hard drives.
  • High Sierra will be released from the App Store in an app named High Sierra Install Assistant, probably instead of a normal macOS Installer app. When run, that app creates and mounts a volume named InstallAssistant containing the Install macOS High Sierra app.
  • The version of APFS in High Sierra release differs from that/those in beta releases.
  • A bootable installer for High Sierra requires a minimum of 10 GB storage. Previously 12 GB was specified.

In the preamble to that note, Apple states that, because APFS on a Fusion Drive “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.”

As High Sierra doesn’t offer any non-destructive method for reverting a volume in APFS back to HFS+, that note next describes two methods for formatting a startup Fusion Drive as HFS+, before installing High Sierra and restoring the volume from a Time Machine backup.

The first reveals that Apple will be releasing High Sierra from the App Store in the form of an app named High Sierra Install Assistant, which can be used to create an external bootable installer (a process which I will document separately) or to install High Sierra. It is likely that this will replace simpler macOS installer apps of previous upgrades.

At present, the second method appears incomplete. Step 3 states:
On a Mac running 10.12.5 or later, use the Terminal app to run the following command:
then fails to give the command before moving on to step 4.

Anyone tempted to format their Fusion Drive in APFS then install the release version of High Sierra on it should think again. It is most probable that Apple has deemed this combination to be unsupported because of continuing problems in this first release version of APFS, not because of problems in converting Fusion Drives from HFS+ to APFS. ‘Unsupported’ could mean that you risk not only losing your data, but possibly even damaging the Fusion Drive.

(Thanks to Garry for kindly providing the link.)