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?