Mac OS X 10.12 Sierra update causes red flashing rectangle behind Apple logo on startup

Posted:
in macOS
After the Sierra update (to 10.12 and then 10.12.1) there is a flash of a red flash rectangle behind the white Apple logo on startup when the progress bar has advanced about mid-way.  Then the progress bar flashes green. On shut down the spinning gear is missing.  I booted my Late-2015 21.5 inch 4K Fusion iMac in Safe Mode but the red flash under Apple logo is not there.  I then booted from an El Capitan OS 10.11.6 external backup and there is no red flash.  So it looks like the red flash is the result of the update to Sierra.  I also noticed that just after the red flash rectangle behind the Apple startup logo the progress bar briefly skips to the end and then back before completing the startup process. This same problem with the Late-2015 21.5 inch 4K iMac has been posted on the Apple Support Discussions forum over two dozen times with no resolution.  Apple Support is unaware of the problem or at least has not acknowledged that the problem is with Sierra and suggests it is a hardware issue.

Late-2015 21.5 inch 4K Fusion iMac 
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