Re: Summary of findings – 2017 iMac (iMac18 ,3) with Fusion Drive issues

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