imac unrecoverable error on start up

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Bart32540 Author

Level 1

9 points

imac unrecoverable error on start up

UnrecoverableError. SecurityAgent was unable to create requested mechanism builtin:prelogin.

How do I fix this?

iMac 21.5″, 11.2

Posted on Jun 6, 2021 8:23 AM

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Answer 1

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Tesserax

Level 10

149,525 points

Jun 6, 2021 2:41 PM in response to Bart32540

It appears that something has gone "south" with the restore of macOS on your iMac. Since you already have TM backups, I would suggest that you consider doing a complete "erase & re-install" of macOS. Then try out your iMac for a few days to see if this issue does not return. If not, then restore your data from a backup.

Ref:

How to erase your Intel-based Mac - Apple Support
How to reinstall macOS - Apple Support

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Answer 2

Bart32540 Author

Level 1

9 points

Jun 6, 2021 10:45 AM in response to Tesserax

Not sure about the model but I think it’s a 2018/9 running Catalina 10.15.7. I was trying to back up an iPad to the iMac and was asked to update (even though I was running 10.15.7) in order to complete the backup. The update seemed to be successful but the backup stalled out and I could not restart the iMac. Then I reloaded the system files from a TimeMacine backup. Since the reload I have been getting the Unrecoverable Error message. I am now attempting, in desperation, to reinstall 10.15.6 from TimeMachine. I have tried a bunch of recommended stuff (P/V RAM, ran disk utility on both HD volumes). I understand there may be a “Terminal” fix but I don’t know how to do that. Many thanks for your help and interest.

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Answer 3

Bart32540 Author

Level 1

9 points

Jun 6, 2021 3:11 PM in response to Tesserax

Did a complete erase and reinstall of OS 10.15.6 from TimeMachine. Everything seems to be working OK. But now I’m afraid to go back to 10.15.7. I am also afraid to try backing up my iPad again. Any further ideas you may have would be helpful. Thanks again for all your help.

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Answer 4

Tesserax

Level 10

149,525 points

Jun 6, 2021 4:14 PM in response to Bart32540

I suggest that you stay at 10.15.6 for a few days. Do not attempt to backup the iPad yet. Then make a complete backup of your Mac. Better still make a bootable clone copy of your Mac to an external drive. I would suggest using Carbon Copy Cloner for this.

With both backups done, then try backing up your iPad. If that goes well, try updating to 10.15.7. With the Mac at 10.15.7, try backing up your iPad again. If all goes well this time, you're done. Congratulations! Should something fail, you now have two backups available to work with.

Unfortunately, I don't have a spare Mac handy running macOS Catalina to experiment with to find out if there is something amiss with the latest 10.15.7 update causing issues with backing up your iPad.

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Answer 5

Tesserax

Level 10

149,525 points

Jun 6, 2021 8:37 AM in response to Bart32540

Try booting up your iMac in Safe Mode to see if it will bring it up to the login screen/desktop. Does it?

Ref: Use safe mode to isolate issues with your Mac - Apple Support

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Answer 6

Bart32540 Author

Level 1

9 points

Jun 6, 2021 8:44 AM in response to Tesserax

Safe mode startup results in the same error message. i.e iMac will not start up in safe mode.

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Answer 7

Tesserax

Level 10

149,525 points

Jun 6, 2021 8:47 AM in response to Bart32540

Which exact iMac model do you have and is it running macOS Big Sur 11.2 as per your tagline? When did this error start happening ... after an upgrade or install of a new app?

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imac unrecoverable error on start up

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