Intel iMac freezes halfway through boot

Question

After a recent power cut, my late 2018 iMac can't seem to complete a normal boot process. About halfway through the booting progress bar, it hangs and doesn't make any more progress.

What I've tried so far:

1. Resetting the NVRAM
2. Resetting the SMC
3. Booting in safe mode (it boots but up is VERY slow), then restarting normally.
4. Booting in recovery mode and doing first aid via Disk Utility (no issues).
5. Booting in safe mode and doing first aid via Disk Utility (no issues).
6. Reinstalling Big Sur Beta (I'm in the beta program). The install never completed because in one of the restarts during install the boot stalled halfway.

I've read through this similar thread here. But it's very old and nothing in there helped.

Answer 1

Even though Disk First Aid has reported no issues, I would test booting from an external drive.

If the problem persists, then you know it's related to some other part of the hardware.

If the problem is not apparent, then the internal hard drive is at fault.

If you have a mechanical hard drive (as part of a Fusion volume), then it's entirely likely that it might be failing.

Answer 2

In the end, I booted in verbose mode and was seeing disk I/O errors. Then I booted in diagnostics mode and saw no issues so I'm guessing the power cut corrupted my boot drive.

So I went ahead and reformatted the disk, and did a fresh install of my systems original MacOS version using cmd+option+shift+R (recovery mode that defaults to the originally installed version of MacOS).

Fingers crossed that the disk isn't failing.