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 My old Nexus 9 tablet was caught in a boot loop: it would boot up, then freeze, reboot, freeze, ... until the battery was exhausted. The instructions at  stackexchange  helped a lot. Here is a summary of what I did: Download the latest OTA file from Google's Developper site . This is a zip file. Do not uncompress it. Reboot in Recovery Mode. For the Nexus 9, press the power and volume up buttons until a menu appears. Choose Recovery Mode. Once in Recovery Mode (recognisable by its Android mascot laying on its back), quickly press the Power Button and Volume Up to enter a hidden menu. Choose "Apply update from adb" On the computer, run "adb sideload <ota_file.zip>" After the update is done, choose "Reboot system now"

 It appears that in some cases the upgrade from macOS Catalina to macOS Big Sur causes a user with Administrator access to lose these admin privileges. One of the main symptoms is the inability to unlock the Security and Privacy System Preferences app. There is a thread on the Apple developper forum that describes the issue and provides possible fixes: Reset the SMC. This actually worked in my case, and I didn't try the other proposed fix. Reboot in Recovery Mode (press Command+R right after turning the computer on), open the Terminal (using the Tools menu) and delete /Volumes/Macintosh HD/var/db/.AppleSetupDone