As reported by MacRumors, the upcoming new Apple File System (APFS) feature in macOS High Sierra, will not support Fusion Drives and will be limited to Macs with all-flash built-in storage. Although iMacs with Fusion Drives were converted to APFS during the beta testing process in the first macOS High Sierra beta, but support has now been removed and will not be available when it is released to the public next week.
Apple has officially confirmed that APFS will not be available for Fusion Drives, and has also provided instructions for converting from APFS back to the standard HFS+ format, with the release of the GM version of macOS High Sierra. 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.”
“Public Beta testers who had an iMac with a Fusion Drive converted to APFS will need to follow a long list of instructions to convert back to HFS+, including making a Time Machine Backup, creating a bootable installer, and using Disk Utility to reformat their iMacs and reinstall macOS High Sierra.
Apple says APFS 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 new Apple File System is an advanced version of HFS+, optimized for solid state drives, offering better security, crash protection, simplified backups and stronger encryption.