Failing reinstall of El Capitan on formatted HD - no Disk Utility (2012 iMac, white screen of death)

[CrossyTM]

Hi
My 2012 iMac (quad-core i5; 12 GB RAM, 1 TB HD) got stuck, white screen of death, as it seems. After tons of repair attempts (with all kinds of startup-key holdings), I backed-up the HD contents via my Mid 2015 PowerBook and formatted the iMac’s HD. I prepared a USB stick with the El Capitan installer, and with the USB stick in the iMac, holding the option key, I could choose the USB stick as startup disk. The Apple symbol shows up and the status bar fills to about 60% when the screen quickly goes black and then becomes completely white - and stays like that forever… I would have expected that El Capitan would install on the iMac or the Disk Utility tool would show up.
What’s wrong? (Btw: Recovery / internet recovery did not work either.) - Desperately seeking help, thanks for any useful advice!

(A similar problem seems to have been discussed like 5y ago at https://www.mac-forums.com/threads/new-ssd-usb-clean-install-issue.331635/ - with a grey screen whereas to me my screen seems white; and I am not aware of any HW problems...)

 

[chscag]

Welcome to our forums.

How did you backup the contents of the iMac with your MacBook Pro and then format it? Did you use Target Mode?

The reason I ask is because your 2012 iMac may have a graphics problem. If the GPU fails, the symptoms you describe will occur.

 

[Rod]

You said you downloaded the El Capitan installer and put it on a USB flash drive and tried to boot from that unsuccessfully.
Did you create a bootable installer on the USB stick or just put the downloaded installer on the USB stick as downloaded?
That will not work. You must use either Terminal or a third party app like DiskmakerX to create a bootable macOS installer from the downloaded installer you got from the app store.
You can still do this on your MacBook and the simplest free app I have found for the job is InstalDisk Creator from macdaddy.io
Once you have that, turn off your iMac, plug in the USB stick and boot holding the Option key. You will then have access to Disk Utility and Installer as expected.
Assuming you don't have a hardware problem you should be able to erase and reinstall the El Capitan OS but I don't understand how you backed up your data.

 

[CrossyTM]

Thank you very much for your contribution chscag!
Yes, indeed. Target Disk Mode enabled me to copy the files and create a disk image and to format the iMac HD. That works fine.
By now, I desparately copied back the disk image, for another attempt: Via Thunderbolt and Target Disk Mode, I'll try to install El Capitan on the iMac HD - In contrast to the Sierra installer which solely would let me select my MacBook Pro for installation, the El Capitan installer offers to select the iMac HD, but: (at least) when the iMac HD was formatted, the installer showed the iMac HD, but said "OS X can't be installed on this disk. macOS isn't installed" (same with the USB stick, of course). Hopefully, with the OS and everything reinstalled from the disk image, I can effectively select the iMac HD for installation of El Capitan... I shall report.
Interesting that the GPU might cause my problem... At least up to the point where the status bar under the Apple symbol at startup is filled to about 60%, the monitor & graphics work fine.

I had run the diagnostics a couple of times - I am not sure whether and to which extent the GPU is tested at that popint... But it showed no problem (except for once or twice, where a memory error (4MEM/62/40000000:…) had been found (allegedly?!), but that occured only once in like 8-10 dignostics runs, and I had furthermore rearranged the 4 memory modules in various ways; no reproducibility of the alleged memory error).

What I also did is let the iMac start in verbose mode (command-V at startup). I took photos from the screen to see whether I can infer something useful from the many outputted lines... but apparently, I don't have the background to properly understand and interpret all the stuff... (and some parts of it may be due to the formatted DH...) - Maybe I'll try again, now that the HD image is reinstalled.

Btw: In the meantime discovered a TimeMachine backup (from 2014...) and tried to reinstall that (HD via USB connected to iMac, hold Option key at startup, select the HD); but after the short lines running along a circle under the Apple symbol ran for some time, the blue screen of death showed up...
I don't know whether that might be related to the last OS X version I had installed on the iMac (which probably was High Sierra - I am not sure whether that might matter and how I could definitely find out which OS X version I had installed at the time of crash.

 

[CrossyTM]

Hi, Rod, thanks for your input.
For the bootable installer I used the instructions from Apple and from Macworld, making use of the createinstallmedia command (in the terminal).
After Option at startup, I have been able to select the USB stick. But no Disk Utility turned up (and no installation took place)... I don't know whether I still made some kind of mistake in this regard...
As to the data backup: I did it by Target Disk Mode, as described in my other reply some minutes ago.

I don't know whether this might be of interest, but
- in the weeks before the crash, the iMac was very slow at times, and that seemed to be due to Firefox (way too many tabs in way too many windows...) (but more than 300 GB of the 1 TB of the HD is free...)
- the crash occurred while being on an olmypus (camera) site in Firefox which was trying to show or install something (if I remember correctly...)
- to me, the iMac seems to be heating up quite a lot; not sure whether that has always been the case or has started only in the weeks (or so?) before the crash, and that is still the case (but I don't know how warm / hot that iMac 2012 model shall / may be during operation.

 

[CrossyTM]

Hi, All
Installation of El Capitan on either a (formatted) USB stick or on the formatted iMac HD (via Target Disk mode and Thunderbolt cable) is obviously excluded: The installer says "OS X can't be installed on this disk. macOS isn't installed" (in both cases).
I decided to reinstall the disk image on the iMac HD. And now? Again via Target Disk mode and Thunderbolt cable, the El Capitan installer (downloaded by and onto my MacBook Pro) made possible to choose the iMac HD (! - yes!) and accomplished the OS installation "successfully" (!!). - However, shutting down the iMac and restarting it did not let the iMac start up as it should, but the original error persists (white screen after abot 60% of the status bar). Only novelty: after some minutes, the white screen vanishes, and the iMac restarts by itself...
Thus: not very much gained...

But:
I looked at the output from the verbose-mode startup; and I had further diagnosis (hardware test):

There are two points I am wondering about...

1. I looked at the lines which show up in verbose mode (CMD-V at startup). And there is something in the iMac's file system which makes me wonder, whether a former OS installation / update went wrong, such that (in a way) two systems are installed in parallel which again creates a problem...
In the verbose output, I have these lines:

> LibBootCache: Unable to open: /var/db/BootCache.playlist: 2 No such file or directory
> Warning: kextd unavailable; proceeding w/o lock for <unknown>
> Warning: couldn't block sleep during cache update
> Warning: proceeding w/o DiskArb
(later: )
> tzinit: New update not compatible or older version: 2020a.1.0 vs 2020a.1.0: No such file or directory
> Sun Jan 31 (more stuff here) Early Boot complete. Continuing system boot.
(later: )
> Notice - new kext com.apple.driver.KextExcludedList, v13.2.1 matches prelinked kext but can't determine if executables are the same (noUUIDs).
(later: )
> unsupported CPU
> unsupported CPU
> unsupported PCH
> HID: Legacy shim 2
> BCM5701UserClient::clientClose
> BCM5701UserClient::terminate
(later: )
> promiscuous mode enable succeeded

In the file system of the iMac, as visible in the Finder, I have the folders
Applications / Library / System / Users
Within "Library", there are about 60 subfolders.
But within "System", there is another folder named "Library" which contains about 97 subfolders - most of them seem to correspond to the subfolders in the upper-level "Library" folder... Is that the way it's supposed to be?? Is it rather a source of the problem? Help by simply deleting a number of files / folders from the HD...?

2. I had the diagnosis (hardware test) run (option-D at startup), and once it (again) showed a memory error: 4MEM/62/40000000: 0x88e61518 . (I suppose this is clearly a RAM error...) Only sometimes this (or a similar) error is diagnosed (in the extended test only, anyway). I am wondering whether one can tell which one of the four RAM modules is the (occasionally) faulty one?
If one cannot tell, I shall go ahead and carry out some dozens of tests, I think, with the one or the other pair of RAM modules removed... (would there be something to be watched out for while doing this, such as at the first restart after a change of RAM?)

I don't know whether the two (potential) problems are independent from one another or whether perhaps the memory problem (2.) caused the OS system problem (1.)...?!
To me it seems best to firstly investigate and solve the memory problem (2.) (which apparently is present)? Or am I heading a wrong way?

Further comments & advice most welcome!