Usually when you install new software on your computer, the installer checks to make sure that you have enough space. After all, if you don’t, how do you expect the installer to complete the installation, right? However, for some reason, Apple’s macOS Big Sur update isn’t checking for install space properly.
What happens is that the installer will proceed with the installation, but since there isn’t enough space, this creates problems. In some cases, the Mac that the update is being installed on becomes unresponsive and can get stuck in boot loops and even have the data corrupt. This seems to be an issue that has been around for a while and despite Apple releasing updates to macOS Big Sur, this is something that the company has either ignored or has yet to get a handle on.
This problem only seems to happen to users who are upgrading to Big Sur from a previous generation update. For those who already have Big Sur installed and are upgrading from one version to another, this isn’t an issue and the installer won’t complete the install and will instead boot back into the operating system.
Do note that if you are upgrading to Big Sur, Apple states that you will need to have at least 35.5GB of free hard disk space, but this does not take into account the 13GB Big Sur installer, which means that just to be on the safe side, having at least 50GB free before making the upgrade is probably a good idea.