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

Someday far in the future, Apple will have settled fully into
using its new macOS filesystem,
, 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

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

Ask Mac 911

We’ve compiled a list of the questions we get asked most
frequently along with answers and links to columns:
read our super FAQ
to see if your question is covered. If
not, we’re always looking for new problems to solve! Email
yours to [email protected] including screen captures as
appropriate, and whether you want your full name used. Every
question won’t be answered, we don’t reply to email, and we
cannot provide direct troubleshooting advice.

%d bloggers like this: