VERY Unusual fault

[Martin Clem]

I do home repairs on Apple Equipment. Someone brought over an iMac 18,2 3ghz 21.5 2017 with kernel panics & hanging during startup( Catalina )
He said it had High Sierra previously.
I could not get it to boot from any external disk except High Sierra install. I used that to format the drive and then I managed to get a copy of mojave to install. That started to crash almost instantly. Kernal panics , restating on it's own and failing to boot at all.
So I used the High Sierra install to format as extended and put High Sierra on it. It's working perfectly??????
What the hell is going on?? All my OS installers are 100% i use them daily. what architecture is not utilised in High Sierra that is failing when i install Mojave or Catalina?? Is the hard drive not compatible with APFS?
There's obviously a hardware problem , any clues??

 

[TwoH]

Could very well be an issue with the HDD - have you ran a test to see if there are any errors with the drive?

 

[DeltaMac]

What type of drive is installed? (spinner)HDD? SSD? Fusion?
Even High Sierra would make the boot drive APFS

Does that iMac still have only the original memory, or has that been upgraded?

 

[Martin Clem]

Normal 1tb sata hdd - no errors
Original ram - passes rember test
Apple Diagnostics - no issues
I’m looking for expert technical help here??

 

[TwoH]

Apple Diagnostics - no issues

What do you mean by this? You ran the simple diagnostic test? Or have you actually checked the HDD for SMART errors?

 

[tyc0746]

My professional opinion...if it's a SATA HDD, then leave it at High Sierra

 

[DeltaMac]

APFS format will work on an HDD, but it isn't happy about doing it! (and you won't be happy with the performance of a spinning hard drive formatted APFS.)

It's not particularly difficult to replace the spinning hard drive with an SATA SSD, and you get a Mac that will actually upgrade successfully to Catalina, or Big Sur, should you want to do that.

 

[Martin Clem]

Jeez!
I’ve installed every operating system going on hdd and SSD. I know very well what I am doing! Brand new iMac with hdd are shipped with Catalina!
why is this iMac only working with High Sierra??
no smart errors...

And btw it is difficult changing the hdd in a sealed unit... 🙄

 

[TwoH]

Could simply be a logicboard fault. By the sounds of it with the kernel panic only happening on certain OSs - it would require a deep dive into the board itself.
Potentially trying to unplug certain parts of it and seeing if it continues with the fault - silly things such as airport/bt card and the headphone/mic.
Have you tried recovery for a fresh 10.14/15 OS too instead of copying over? Justto be certain.

FYI I wouldn't say its "difficult" to replace the HDD in any of the sealed iMacs - it looks "scary" to anyone who hasnt had experience opening one before, and thats it.

 

[JustMartin]

You could try posting some of the log here to see if people recognise any of the panics. Presume this also happens when you boot in safe mode?

 

[Fishrrman]

My guess:
Firmware (as opposed to "hardware")...