2017 iMac keeps suddenly shutting down

Follow up a month later: so far so good - since the Apple Store "re-fused" my Fusion Drive (or whatever they did), the sudden shutdown issue has not reappeared. Everything seems to be working as it should, and the computer by and large feels significantly faster than perhaps it ever has since I bought it a year ago.

But I did also shell out for Apple Care, which I've never done before. It's understandable that I'm a little worried about the stability of this machine going forward!

Etresoft, thanks again for the help on figuring out the nature of the problem.

Oh man, this sounds bad; thanks for your help with this. I'll attach the EtreCheck report from a couple of weeks ago that included the warning about the failing hard drive - this was when the crashes first started, although as you suggested earlier, it's unclear if the SSD issues are related to the sudden shut downs.

Inside EtreCheck, is there an archive of earlier reports? I've got several of them saved separately as PDFs, but I can't find any of them dating from before these crashes started (nor am I sure if there were any). I'm looking through what records I've got to try to figure out if this "missing" SSD was ever working for me in the eleven months since I bought the machine.

EDIT to add: I don't suppose there's any way to dig into Time Machine to see if the SSD was present or absent at different times over the past year?

Thanks - yes, it does seem similar to power supply issues that others have had. I’ve left the computer turned off overnight, and have now turned it on again - I’m just going to let it sit for a while to do a Time Machine backup. Some of the links below report having the issue very sporadically, with the computer being basically unusable and then not having an issue for weeks. I’ll see what I’m dealing with.

Sigh. This is when I regret buying a big desktop and not owning a car. Getting to a place that can fix this is going to be a hassle.

A few links for what seem to be similar situations:

SOLVED: Apple STUMPED - iMac shuts off randomly, daily to multiple times/hour - iMac Intel 27" EMC 2546 - iFixit

iMac shuts down randomly for no apparent reason

new 27" imac 2018 has been having random intermittent shutdowns for 3+ months (my iMac shuts down for no reason and won't restart unless unplug with full hard reset - any technical thoughts on how to resolve?

(Lack of) progress report: the computer continues to shut down, apparently randomly - today it crashed twice when I tried to paste something in Excel, but the first time it crashed today was just when I was typing in Notes. Some days I spend a couple of hours on the computer with no issues. Other days it powers off within a few minutes. I believe it always powers off in response to a key stroke - if it's just left turned on, it doesn't seem to shut down.

Etre Check attached; for what it's worth, this computer, not quite a year old yet, has never really run that well - often slowing down or beach balling, and and it was a far smaller improvement over my previous 2009 iMac than I expected. I know I've just got the minimum RAM, but it still sometimes beachballs just when I'm opening Safari, and it's often faster to find a file through Finder than it is to use Spotlight, even if only a couple of apps are open. I've suspected buying more RAM would be throwing good money after bad.

I haven't been able to get to a Genius Bar yet, but hope to soon, but it'd be great to be able to give them more definite information pointing to power supply problems (if that's what it is). Is there anything I can specifically point to to help them diagnose the problem?

You will need to restore it to a factory configuration. Then, if it fails again, you can get it diagnosed and repaired. In this situation, the only logical course of action is to restore the configuration and then see what happens.

OK, I'll try that. Just to confirm before I embark on this process: I assume I need to ...

1. Back up everything (I've got two Time Machine drives going, plus SuperDuper clones I normally do every three days).
2. Wipe the system, reinstall new OS: I plan to follow the iMore steps for wiping the system.
3. See if the SSD is functioning on the "new" system. How should I test this? I can go into About this Mac / Storage, and see if it's 2TB or more than 2TB. Or: right now, when I look in About this Mac / System Report / Storage, and look at my Macintosh HD, I see only the HDD listed - there's no entry for the SSD. Should there be one? Are there other useful ways to see if the SSD is working?
4. If it's not working: contact Apple Support, and quite likely eventually take it to an Apple Store;
5. If the SSD is now functioning: reinstall my system from Time Machine.

Gulp. Last time I restored from a backup, just three months ago, it took days for the system to reinstall everything, and then maybe a week for iCloud to sort itself out.

For my curiosity about when this got scrambled: remember this thread started because of a sudden shutdown issue (which, oddly, hasn't happened in a couple of days). I still wonder if the sudden shutdowns and the detached SSD are related or completely distinct issues.

(I'm curious how common a problem this detached SSD is. I haven't been able to find any other instances, but perhaps my Google skills are lacking; however, there are very few unique problems. And there could be many people like me, who get a new computer with a Fusion Drive and go through their years with the computer thinking "Huh, this isn't as good as I expected", never realizing there's a deeper problem.)

Thanks again for all the help.

Whoah - thanks. What part of the EtreCheck report shows that the SSD part isn't being used? I had no idea - it would certainly explain the slowness, and why my 2017 machine has never seemed much faster than the older 2009 one.

I haven't knowingly done anything with the Fusion Drive. Two points that come to mind: first, when I set up the system a year back, I believe I used a Time Machine migration from my older 2009 iMac - but surely that wouldn't have decided to ignore the SSD? Second, I had a big crash about three months ago, and getting myself set up again took a lot of time with Apple Support. I believe the eventual restore was from Time Machine, although perhaps it was from a SuperDuper clone. But the slowness issues predated that. The sudden-shut-down issue is a couple of weeks old.

Occasional Etre Checks will also tell me my Fusion Drive is at risk of imminent failure, but that didn't come up with this check.

(I'd like to link to posts about my crash a few months ago, but apparently Apple Support Communities has once again changed ways to find older material ...)

EDIT: ah, found the older thread, which doesn't have much that's helpful; it looks like I reinstalled Mojave, and then restored eventually from either Time Machine or a Super Duper clone:

https://discussions.apple.com/thread/8559505

More on whether or not my SSD has ever been working: I've got some old crash reports, from a couple of weeks after I bought the computer, and at the bottom, they've got "Serial ATA Device: APPLE HDD ST2000DM001, 2 TB" - just the HDD listed, no SSD. Does that mean anything?