Apple has fixed a weird issue with macOS Big Sur that previously allowed people to install operating system updates even if they didn’t have enough space on their Mac. Unsurprisingly, installing updates without space can cause various problems in the system.
If you have the macOS Big Sur installer on your system, please check if you have updated your Mac and all its applications to the latest version of the operating system. Then follow the instructions here to confirm which version of Big Sur InstallAssistant you have, in case it is still waiting to run on your Mac. If you are not using 11.2.1 (20D75), please delete it and reinstall the macOS Big Sur update package.
After completing this operation, if there is not enough hard disk space, you will not be able to install macOS Big Sur. If not, you have several options to free up space. First make a full backup of your system. If you know where you usually save some large files (such as your download folder), start from there; delete the content you no longer need and move the rest to the cloud or external storage.
If you need help, please use the storage management application on your Mac to move your files, photos, and messages to iCloud; delete movies and TV shows you’ve watched from your Mac; automatically clean up the trash bin; and confirm You no longer need files stored on your Mac.
Or use the tool to manually search for files that take up a lot of space. Simply click on the various categories in the sidebar (applications, documents, iCloud drive, information, and trash can) to view the different files that take up all the hard drive space.
If you have tried to install macOS Big Sur but there is not enough space and the update crashed your system, you have several options to delete it and save your data. They are a bit deep, but most people should be able to reach them. If you have FileVault enabled, you will need to install macOS Mojave on the external storage and boot from there to free up space on the Mac’s primary hard drive, or use a second Mac in target disk mode to mount your disk and release it Space. Complete the installation of macOS Big Sur.
Otherwise, you can use the terminal application in macOS Recovery to delete the files, or use one of the two techniques above to free up space and complete the Big Sur installation.
For this reason, I always recommend backing up your data before major operating system updates. If you don’t need anything on your Mac’s hard drive because you’ve backed up everything, troubleshooting becomes easier. Simply boot into your Mac’s recovery mode, erase your entire hard drive, and reinstall macOS. You can then use Migration Assistant to get all your files back, even as you figure out how to free up enough space on your system to install macOS Big Sur.