gbol Said:
“Big Sur Hangs at Startup: I installed Big Sur, but after the download, the computer never gets past the black startup screen. I keep getting a message there was an error starting up, and to press a key or wait to start up again. I am caught in this loop. Pretty frustrating.”
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Trojan (Looping Item):
That would seem to me to be a looping item - a trojan of sorts that keeps repeating itself.
Boot into Safe Mode:
Hold Down the shift key. If you can log in:
First, clear your Temporary Internet Files - Apple Communities
Then, Scan your Mac for Malware:
Use MalwareBytes for Mac --- This is software that searches for malware/adware. So, scan with it and then remove what is found. Once removed, uninstall MalwareBytes for Mac. Then restart the Mac.
Downloads:
- Malwarebytes Anti-Malware for Mac
- Malwarebytes Uninstaller
If Neither Works - Reinstall the macOS:
1. First, Back up your Mac:
If you can boot in to Safe Mode, then create a Time Machine backup of your Mac, so that you can have something to restore your Mac from, should anything go wrong with the install.
2. Then, Reinstall the macOS:
Nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to reinstall macOS from macOS Recovery - Apple Support