Before we get started
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The good news, which you probably already know about, is that macOS High Sierra will be released six days from now. The bad news is that the new Apple File System will only work on Macs which have built-in all-flash storage.
To put it differently, it will not work with Mac minis and iMacs that boast Fusion Drives.
During the beta testing phase in the initial macOS High Sierra beta, Macs using Fusion Drives were switched over to APFS, but in later betas support was stopped and never re-implemented.
When it released the software’s Gold Master version, Apple officially announced that APFS will not be released for Fusion Drives. The company offered instructions on how to convert to the conventional HFS+ format.
If you are a Public Beta Tester and your Mac uses a Fusion Drive that was converted to Apple File System, there’s a long list of things you will have to do to make the transition back to HFS+. This includes setting up a bootable installer, creating a Time Machine Backup, reformatting your Mac using Disk Utility, and finally re-installing macOS High Sierra.
Earlier this month Apple released a support document explaining compatibility. It clearly stated: “Fusion Drives and hard disk drives aren’t converted.”
Apple added that there will not be support for APFS on Fusions Drives in the first release of macOS High Sierra. This hints at possible support for Fusions Drives in later releases, after all the bugs have been ironed out.
APFS offers optimised performance on solid state drives and is more modern than HFS+. It is also secure, safe and offers stable snapshots, secure document saves, crash protection, strong native encryption and simpler backups.
With features such as instant directory and file cloning, sparse file writes, fast parallelised metadata operations, and fast directory sizing it is also bound to offer better responsiveness that HFS+.