Resolved iMac 18.3 -27" -2017 @Ventura: no normal shutdown possible - no macOS installation possible

[LuckyOldMan]

Recently, certain functions on my iMac 18.3 have changed in a strange way.

1).
The iMac can no longer be turned off going through the Apple icon at the top left -> Turn off, but restarts instead.
The only thing that helps is a hard shutdown using the PWR button.

Turning off by using the selection menue on the logon-desktop (Turn off - Restart - Hibernate) only leads to a restart too.
Restarting via the menues - either Apple-icon or logon-desktop - works normally.

All attempts using NVRAM reset, SMC reset did not improve the situation.

2).
Installing macOS Ventura, Monterey etc. only goes up to the 2nd phase of the installation - after rebooting again, the macOS installer suddenly disappeared and is no longer displayed in the Apple picker via ALT. Only already installed/existing macOS were displayed.

Installing a new macOS on the iMac data storage device is only possible by using an external case and then connecting it to the iMac (USB-C).

Once installed the data storage back into the iMac, booting is possible without any problems.
But adding another macOS while the disk is installed again is still not possible.

Everything else works as usual.

Any advices, tips on how to solve the problems would be appreciated. Thanks.

 

[Fishrrman]

What year was the iMac made?
What kind of drive is inside?
Did you ever add or replace RAM?

 

[LuckyOldMan]

What year was the iMac made?

As posted late 2017.

What kind of drive is inside?

Big upgrade 6/2023 from original FD (1TB HDD + 32GB Blade) to NVMe-SSD WD_Black SN770 & Sata-SSD WD Blue SA510.

Did you ever add or replace RAM?

Yes - in 2/2023 to the existing 2x 4GB I added 2x 16GB.

So everything worked fine until beginning of may '24.

 

[Fishrrman]

I'd be suspecting either the replaced drive or the added RAM.

You might take the 16gb DIMMs out for 2-3 days, see if that changes anything (only factory RAM installed).

 

[LuckyOldMan]

the replaced drive

Which one do you have in mind - the NVMe-SSD or the Sata-SSD?

16gb DIMMs out for 2-3 days

I did that already 'till vesterday then I put the two 16GB DIMMS in again.

The two 16GB DIMMS worked fine for more than a year.
But anyway - i'll give it a try again & take them off.

 

[Bigwaff]

Big upgrade 6/2023 from original FD (1TB HDD + 32GB Blade) to NVMe-SSD WD_Black SN770 & Sata-SSD WD Blue SA510.

Did you turn the two SSD into a Fusion Drive after install? If so, you’ll want to unwind that.

 

[Fishrrman]

Waff is right.

Do a COMPLETE BACKUP.

Then, UNfuse the fusion drive.

It's possible that one of the blade SSDs has "gone wonky" on you.

 

[LuckyOldMan]

Did you turn the two SSD into a Fusion Drive after install?

Yes - I did it, but only for testing purposes, not for long-term use. Simply to show that an NVMe SSD can also speed up a SATA SSD.

The problems described have been solved: the iMac can be switched off normally again and macOS can be installed again.
The reasons were apparently incompatibility of the 2x 16GB RAM with the 2x 4GB RAM.
It worked perfectly for a year, but suddenly stopped.

Each RAM pair works on its own - whether 2x 4GB or 2x 16GB. I don't know yet why it is like this.

But it had nothing to do with the data storage devices - in whatever configuration. These part works fine - I just changed from the WD_Black SN770 to a SN850X - even better.

Thanks both of you for your input.