iMac constantly reboots during startup

These notes about Windows are part of explaining the whole story, so please keep reading. I was using my iMac yesterday with no problem, initially using Boot Camp and Windows 7. In Windows, I installed NordVPN, then Audacity, while some Windows updates were working in the background. I waited for everything to complete installation, and all was good. I typed music.google.com into Firefox, pressed Return, and the entire screen turned solid royal blue. A few seconds later, the blue screen was pinstriped with vertical black lines. Nothing else was visible; no letters, no cursor, no mouse. There was no way for me to do anything.

I held the power switch in for several seconds, and the iMac turned off. After several more seconds, I turned it on and it started as a Mac. The gray apple appeared, the progress bar was visible, and the screen turned white as if it was about to go to the login screen. But the login never appeared. After an unmeasured number of minutes, the screen went black for a few seconds and rebooted. And the same normal progression of events ensued, only to go black and reboot repeatedly.

I powered down manually and tried several troubleshooting procedures, each time having to power down manually. I used the tips from https://www.macworld.com/article/2018853/when-good-macs-go-bad-steps-to-take-whe n-your-mac-wont-start-up.html Before you click that link to learn more, these are the procedures I tried from startup:

- Holding down Option key allowed me to choose how to boot: Macintosh, Windows, or Macintosh Recovery. Choosing Macintosh or Macintosh Recovery, same reboot loop problem. Choosing Windows, I got the swirling Windows opening screen, then the uncontrollable blue & black pinstriped screen.

- Running Disk Utility, holding down Command-R. I wasn't able to open Disk Utility. Reboot loop still persisted.

- Running Safe Boot with Verbose Mode, holding down Shift-Command-V. Procedure was successful, but reboot loop was still present.

- Running Single User Mode, holding down Command-S, then entering fsck -fy . Procedure was successful, but reboot loop was still present.

I have run out of DIY ideas. Any help is appreciated. Thanks.

iMac, macOS High Sierra (10.13.6), Windows 7 in Boot Camp

Posted on Aug 15, 2018 1:06 PM

Top-ranking reply

Kappy

Level 10

362,007 points

Posted on Aug 15, 2018 3:29 PM

You need to reinstall macOS. There are two ways: Install over the current system; or, erase the drive and reinstall. You will need a good and current backup for the latter because the drive will be erased.

Reinstall El Capitan or Later Without Erasing Drive

Please be sure you back up.

Restart the computer. Immediately after the chime hold down the Command and R keys until the Utility Menu appears.
Select Disk Utility and click on the Continue button.
Select the indented (usually, Macintosh HD) volume entry from the side list.
Click on the First Aid icon in the toolbar. Wait until the Done button activates, then click on it.
Quit Disk Utility and return to the Utility Menu.
Select Reinstall OS X and click on the Continue button.

Also, see this User Tip: Basic Steps for Upgrading OS X.

Install El Capitan or Later from Scratch

If possible back up your files.

Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
Select Disk Utility from the Utility Menu and click on Continue button.
When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
Click on the Erase icon in Disk Utility's main window. A panel will drop down.
Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
Click on the Apply button, then wait for the Done button to activate and click on it.
Quit Disk Utility and return to the Utility Menu.
Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

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iMac constantly reboots during startup

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