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iMac endless restart loop, unable to enter Recovery and Safe Modes

Two days ago I asked a question about my iMac failing to upgrade to Big Sur while restarting, but now I'm trying to diagnose whether or not my iMac is experiencing a more concerning hardware or software issue.


TL;DR my iMac will restart endlessly after a power outage or manual restart, but fortunately I can get it to boot up normally if I use the Startup Manager. During restarts, whenever I attempt to enter...


Recovery Mode: does absolutely nothing - Apple logo appears as if I'm not holding the keys down

Safe Mode: abnormal behavior - causes an instant restart as soon as the Apple logo appears

Diagnostic Mode: reports no errors then offers restart or shutdown as usual

Startup Manager (Option): selecting the macOS installer just resumes the restart loop, selecting my startup disk boots it normally


I am trying to install macOS Big Sur because I have been experiencing very frequent crashes with my software, and working with its developers has led nowhere but presuming a hardware issue on my end.


Troubleshooting other help forums suggest a kernel issue, but I don't know enough about hardware to verify this, and the restart issues don't really point to a kernel panic. I have already reinstalled macOS Catalina (which is possible oddly enough, and not Big Sur) but all these problems persist.


Help would be very much appreciated.

iMac 21.5″ 4K, macOS 10.15

Posted on Apr 21, 2021 12:52 PM

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Question marked as Top-ranking reply

Posted on Apr 21, 2021 1:26 PM

Please refer to the troubleshooting steps in Startup Screens Troubleshooting Steps, if you get through all the steps adn it still won't start up, the next step it to take it to your local AASP to be professionally diagnosed. You can use the AASP Locator to begin finding one if necessary .

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iMac endless restart loop, unable to enter Recovery and Safe Modes

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