I can not start my iMac 27 "by the end of 2012 (and do not let me install, run diagnostic tests, etc.) …
Ok … I will try to stay as brief as possible …
By the end of 2012, the iMac from 3.4GHz to 8MB was working fine – started having random reboots last year – was running under High Sierra
Tried a Mojave update – failed – and iMac would no longer boot / keep the power cycle
It has been repaired by an Apple Authorized Repair Center. They managed to extract all the data from the Fusion disc (128GB original SSD / NVME hard drive + 1TB hard drive). They tried to reinstall the operating system (I think they put the original Snow Leopard on it). But did not stop doing it again.
He told me that it was probably a fault of Logic Board that would cost $ 800 or that I could go with them for recycling …
I decided to try to save the machine. I had read that the iMac PSU cards at the end of 2012 often posed this kind of problem. So I brought home iMac, I learned to open it (iFixit). PSU board for $ 100 and has it equipped.
Do a quick test – and it started! Hurray.
At this point, I was thinking of giving the machine a little help. So I bought a 1TB SSD drive (Samsung EVO 860) and replaced the hard drive with the SSD drive. I was expecting this to kill the configuration of the merge drive – and that's what happened – but I was ready to install Mojave from one place. external USB drive that I had created.
It did not seem to work – but when I tried the Internet recovery mode, it remade the fusion drive, installed High Sierra – and started! So, was super happy, machine-sealed screen and had a new working iMac.
So then … I tried to install Mojave again (10.14.4) … seemed to start with (1 min) then crash … then did not start and did not even get to the utilities of disk or Terminal with my Mojave USB Installation Disk
So I thought that the configuration of the merge disk was perhaps the cause – and so if I could reset it – maybe merge the disks and install OS X on NVME and use 1 TB for applications and user data. .
I bought a Firewire cable (and a new Mac Air book …) and used the Target Disc mode – I could see the drives – I had done first aid – without error. I've tried many different startup options – in the end, I deleted them to try to delete the merge configuration.
All I did was not helped to overcome a blocked progress bar on the Apple logo.
I can not perform Internet Restore, USB Recovery, Paragon Hard Disk Manager USB Drive, Apple Diagnostics (via the Internet) – I get an error about Unable to load EFI with cmd-D
So I figured if it had nothing to do with my internal disks (which should be blank) – maybe it's his memory … so I tried to remove a 4GB bank – the same results. tried other … same results …
The only thing I can enter is the single-user mode – but I do not know what to do to help ….
Before I start to believe that this is a logic board problem, I really want to rule out any software / firmware issues – or test HW and make sure everything works fine – but remember – I do not have OS installed for the moment.
… update …
things get worse – I do not even see drives in target disk mode anymore 🙁 – the host displays the Firewire symbol – but no drive appears on my Mac Book Air (which I just upgrade to 10.14.5 – I hope this did not cause that).
If I use the single user mode (with the USB key 10.14.4), I see the drives. are there (with ls / dev / disk *) – but I can not seem to run fsck on them (only the "root" USB)
What should I do in single user mode to be able to a) check other drives, b) view them in target disk mode, please?
On another thread, Apple Community talks a lot about the latest firmware bug Mojave EFI Firmware 224.0.0 – for example. Stop the operation of Cmd-D diagnostics:
Error: 0x8000000000000003, Unable to load "EFI / Drivers / TestSupport.efi"
I get that too, so I can not easily check what works and what does not work
p.s.I have changed my RAM tonight – NO change.
Thank you for any help