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I have my Late 2015 iMac running on El Capitan. I tried to upgrade it to Big Sur but the update went in a loop, restarting over and over with the Apple logo and the progress bar. Turning off-on did not help. With difficulties, I had to erase the HHD and reinstall El Capitan.

I tried the above one more time, and then two times trying to install Monterey for the same outcome. Then I tried (twice) to install Monterey from a bootable installer. On both occasions, about half way, the screen went black and the computer restarted.

I cannot understand what's wrong. It also puzzles me that Disk Utility shows that the Mac (which is supposedly standard) has both Fusion HDD 2Tb and a 128GB SSD on board. How can this be?

Lastly, this iMac malfunctioned two months ago. It ceased to boot and there was some strange error -- Disk Utility did not see the HDD until I restarted the Mac for a few times. Only the SSD was visible. However, I managed to recover most of the files from Terminal and reinstall the OS. Disk Utility never showed any disk errors since than.

Any ideas how to update the macOS please?

UPDATE:

  1. I was able to update to High Sierra.
  2. Here is what diskutil Terminal command and Disk Utility show (I am confused why these are different). enter image description here
Vlad Vysotsky
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  • Can you add to your question the results of terminal command : diskutil list –  Dec 03 '21 at 06:10
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    You probably need to jump via High Sierra. On phone so can’t find link right now – Tetsujin Dec 03 '21 at 06:18
  • @Tetsujin Good idea. There is the link : https://support.apple.com/en-us/HT211683 –  Dec 03 '21 at 07:03
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    Thank you @Tetsujin. I followed your suggestions. It seems I am finally on a right track, I will try to install OS Monterey now. But the extra SSD is still confusing... – Vlad Vysotsky Dec 03 '21 at 17:29
  • Re your update - you have split the Fusion Drive. See https://apple.stackexchange.com/questions/394356/is-it-worth-unsplitting-my-fusion-drive and https://apple.stackexchange.com/questions/277634/how-can-i-fix-or-restore-my-3tb-fusion-drive-macos-install-after-boot-camp-parti – Tetsujin Dec 03 '21 at 18:26
  • @Tetsujin I see, thank you!

    1. Does it make sense to go ahead and merge the disks? I must add that surprisingly, sometimes Disk Unitiliy shows both HDD and SSD while sometimes is shows only the HDD.

    2. Installation of Monterey over High Sierra did not go through. Does it make sense to try installing Big Sur instead? All of this is very time-consuming, so I am considering to simply bring the iMac to Apple Genius Bar and see what they can do.

     – Vlad Vysotsky Dec 04 '21 at 01:40
  • P.S. 1. But the diskutil command still shows split disks. – Vlad Vysotsky Dec 04 '21 at 01:53
  • The SSD may be failing/intermittent. The Mac will work without it, but will be slower [& that generation of iMac are slow enough anyway] Depending on your finances, you would gain a great deal of performance by replacing the old Fusion drive with a single SSD large enough for your needs. Fusion drives were a stop-gap measure when SSDs were too expensive. Taking iMacs apart is not for the faint-of-heart, so you might be best getting a pro to do the whole job & put an OS on it afterwards. See https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+Retina+4K+Display+Hard+Drive+Replacement/64107 – Tetsujin Dec 04 '21 at 10:31
  • @Tetsujin Thank you again. You were right, merging the disks did not work. They confirmed at the Apple store that the SSD is not working. And they could not install anything above High Sierra too. I might be upgrading the HDD to SSD as you suggested, depending on the costs. – Vlad Vysotsky Dec 06 '21 at 16:56
  • Wish you luck on the new drive. That Mac can definitely run current macOS, Monterey. See https://everymac.com/systems/apple/imac/specs/imac-core-i5-1.6-21-inch-aluminum-late-2015-specs.html – Tetsujin Dec 06 '21 at 17:39

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