Should you upgrade your Fusion Drive iMac to macOS High Sierra?

Users are concerned with Apple new APFS format.

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Someday far in the future, Apple will have settled fully into using its new macOS filesystem, APFS, and Macworld readers will stop being nervous and confused about it. Until then, I continue to receive queries in our inbox. APFS is a robust, more efficient, and futureproofed filesystem that has a lot of advantages. But the transition isn’t complete by any means.

Reader Bill emailed wondering about the safety of upgrading his iMacs to macOS High Sierra, since each has a Fusion Drive, Apple’s hybrid of SSD and hard drive. He notes that he’d read Apple’s software engineering chief, Craig Federighi, had said at some point that APFS would come to Fusion Drives. (Federighi replied to a Mac user’s query via email, which that user then publicized.)

It’s not unsafe to upgrade, because High Sierra leaves Fusion Drives in HFS+ format. However, I’m assuming that Apple will release an update to High Sierra at some point that sweeps in Fusion Drives, and that installing that 10.13 update will automatically and without asking convert your HFS+ startup drive to APFS. This was the case with the release version of High Sierra, which upgraded SSD boot drives to APFS without prompting or a way to avoid it. (The beta releases of High Sierra offered a checkbox to opt out.)

That could be problematic, because if Apple’s Fusion Drive version of APFS isn’t perfect, you could be left with a mess. It may be worthwhile for that reason alone to stick with Sierra until such a point as Apple releases the Fusion Drive APFS update, and then wait to hear from online reports about how well that goes.

Bill also asks about his external Time Machine volumes. High Sierra doesn’t convert anything but a startup drive that’s an SSD to APFS, so a startup hard drive or Fusion Drive remains untouched, as well as any external drives, no matter what type of drive they are.

While you can manually upgrade drives via Disk Utility to APFS, do not upgrade Time Machine volumes. Apple weirdly released macOS with APFS without figuring out how to manage Time Machine under APFS. Switching from HFS+ to APFS destroys your Time Machine backup, as I write about here.

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