On 31/01/2026 05:24, Paul wrote:

On Thu, 1/29/2026 6:15 AM, David B. wrote:

Summary of findings ? 2017 iMac (iMac18,3) with Fusion Drive issues

I?m running a 2017 iMac (iMac18,3) which originally shipped with a 2TB Fusion Drive.

Based on behaviour over time, my assessment is that the SSD portion of the Fusion Drive has failed, while the 2TB spinning HDD remains functional.

Current setup and observations:

macOS Ventura is running from an external 1TB SSD and is stable and performs well

Linux Mint 22.3 is installed on the internal 2TB HDD (spinner) and also runs reliably

The same hardware shows instability only when macOS interacts with the Fusion Drive configuration

Kernel panics previously observed (including namespace 2 / SIGBUS-style errors) ceased once macOS was moved entirely off the internal Fusion Drive

This strongly suggests a partial Fusion Drive failure, which macOS/APFS handles poorly when one physical store (the SSD tier) becomes unavailable or unreliable. Linux, treating the HDD as a simple block device, is unaffected.

Diagnostic/benchmark tools show reasonable performance for the active boot device, but they do not expose intermittent Fusion metadata or tiering failures.

Conclusion:

CPU, RAM, GPU, and logic board appear sound

Internal HDD is serviceable

Fusion SSD blade (or its metadata) is the likely root cause

Running macOS from an external SSD and Linux from the internal HDD is a stable and practical workaround

Posting this in case it helps others with similar 2017 iMac/Fusion Drive symptoms.

You'll need the kit of materials to adhesive the front glass back on,
after you cut it away to get inside the unit.

https://www.youtube.com/watch?v=jNlJRTRC_GI

They depop the connector, on the motherboards
that don't have Fusion as a feature. That video still wasn't
good, in terms of showing where all the storage connectors are.

Wiki has a description, but you really need to read some of the references
to see how it works. It's tiering and doesn't have sufficient notions of
RAID to be called RAID. You can artificially promote files, so they
end up on the SSD. It also does not look necessarily, like an Intel technology (like SRT). It's not just some rebranding.

https://en.wikipedia.org/wiki/Fusion_Drive

https://arstechnica.com/gadgets/2012/11/achieving-fusion-with-a-service-training-doc-ars-tears-open-apples-fusion-drive/

It's a shame this will achieve a "Swiss Cheese" on a failure. I just
think features like this, and just about all the Intel attempts to
do similar, are garbage. Why ? Because the amount of support questions,
hair loss on failure, who gives a fuck if something is fast, if it
isn't reliable ? Reliability comes first, fast comes second. There's no
way we want customers asking AI for advice on "how to step out of
a pot of boiling water". Just don't go into the boiling pot in
the first place, that's my advice.

One repair tech on Reddit, said he'd done lots of Fusion repairs,
but he'd never had to open the one that just comes with some
SSD and nothing else. So he doesn't know what the connector looks
like, from not having to do maintenance on the actual good machine
to own.

Doing tiered, preferentially writing everything to the faster
drive all the time, doing promotions and demotions, that puts a lot of wear on the fast drive. And really, if you were going to do that,
the fast tier should have been Optane. As it could handle
a lot more write cycles than NAND. It runs a bit warm, so it
would have to be laid out to share a bit of cooling air.

Thank you so very much for your comments, Paul.

I'm not sure if I've asked you before. Have you ever explored here?

https://x704.net/bbs/

Maybe it's another place where you could share your invaluable knowledge.

--
Kind regards,
David

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