Update to Big Sur from Catalina fails...need to troubleshoot and resolve
Relatively young iMac...i5 processor, 8GB RAM, 1 TB HD...nearly empty.
Always freezes with '12 minutes' remaining...How do I finish this upgrade?
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Installing the Big Sur upgrade could easily take much longer than an hour depending on the specifics of your Mac. Are you giving the machine time to do its thing?
If you feel the Mac truly is stalled during the upgrade, start over and try the Upgrade while booted in Safe mode.
• Shut down your Mac.
• Disconnect all external peripherals, drives, printers, docks, etc.
• Boot in Safe mode (press the Shift key when restarting your Mac and continue to hold it until you see the Apple logo and progress bar.)
• Run the upgrade while booted in Safe mode.
With any luck this may get you through the upgrade.