I had been putting off updating my mid-2014 because I didn't want to go through this process again, but I updated without thinking. I now have Big Sur 11.6.5 and the steps for this no longer work because the drive can't be mounted as writeable. I've tried a myriad of things after scouring the web and haven't found anything that works. When I try to delete/rename the AppleThunderboltNHI.kext, it keeps telling me it's a read only file system. I have verified that csrutil and authenticated-root is disabled. Any suggestions by anyone?

I was having the random shutdowns after upgrading to Big Sur on my MBP mid 2014 this afternoon. I was running Mojave previously. I followed all the steps from sf98723 but was experiencing the same issues as mergenc with the 'Your computer restarted because of a problem" error. I restarted in recovery mode and ran both of the following: csrutil status csrutil authenticated-root status ...both were enabled. Not sure how the authenticated-root got re-enabled, but I disabled the authenticated-root and rebooted and everything is working. So far no unexpected shutdowns. Thanks for the solution.

I followed the instructions by sf98723 on Catalina while trying to update from 10.15.4 to 10.15.7 but when I try this part:  bless --folder /Volumes/[MacOS your custom stuff here]/System/Library/CoreServices --bootefi --create-snapshot I think the snapshot might be new for Big Sur, so if you're on Catalina, I think there may be a different set of instructions to get around the AppleThunderportNHI issue.