This could get annoying.
Apple has released a technical note describing how the new Apple File System (APFS) feature will be limited to Macs with all-flash built-in storage, which means it won’t work with iMacs and Mac minis that include Fusion Drives.
The Fusion Drives were converted to the APFS format during the first beta test of MacOS High Sierra, but support was removed in subsequent betas and not reimplemented.
Following the release of the High Sierra Golden Master, Apple has confirmed APFS will not be available for Fusion Drives and has provided instructions for converting from APFS back to the standard HFS+ format.
As such, users who participated in the public beta and had a Mac with a Fusion Drive converted to APFS will need to convert their drives back to HFS+ format. This involves making a Time Machine Backup, creating a bootable installer, and using Disk Utility to reformat their Macs and reinstall macOS High Sierra.
Apple on September 5 published a support document confirming compatibility. When customers with an all-flash machine upgrade to macOS High Sierra install the update next week, their drives will be converted to AFPS. Apple explicitly says “Fusion Drives and hard disk drives aren’t converted.”
The company has stated that the APFS format will not be supported on Fusion Drives “in the initial release of macOS High Sierra,” which suggests support could be added for Fusion Drives at a later date after lingering bugs are worked out.
The Apple File System is a more modern file system than HFS+, is optimized for solid state drives and offers crash protection, safe document saves, stable snapshots, simplified backups, and strong native encryption.
It’s also more responsive than HFS+ with features like instant file and directory cloning, fast directory sizing, high performance parallelized metadata operations, and sparse file writes.
The full release of macOS High Sierra is due on Monday, September 25th.
Stay tuned for additional details as they become available.