Updating boot support partitions as required
To be clear, what we’re describing here is a Mac that is stuck on an all white screen with no logos, no progress bar, nothing, it’s just a blank white display.There are vairous potential causes for a white screen to appear on a Mac during startup, so we’re going to cover a wide range of troubleshooting tips that may resolve the issue.Has your Mac ever been stuck on a white screen during boot? Did you resolve it with the tips above or with a different solution?
If the other approaches are failing, you may need to reinstall Mac OS system software.
Not all steps may be required to resolve the problem, you may be able to simply reboot into Safe Mode and reset NVRAM and fix the white screen issue, for example.
This is quite easy; simply reboot into Safe Mode on the Mac with a properly timed shift key press.
Make sure its not the same partition which contained your previous Windows version which has been now upgraded to Windows 10.
When the installation will complete, you'll have an already activated fresh Windows 10 as you have already associated your product key with Windows 10 in step 1. Now you have two Windows 10 in your computer: one which was upgraded from Windows 7/8.1 and the other which was clean installed.