Apple today released a revised macOS Big Sur 11.2.1 update that fixes an issue that could cause installation issues on Macs that did not have enough storage space available. The original update was not checking to ensure there was enough free space available before starting the installation process, causing possible data loss.
According to Mr. Macintosh, the new update checks to make sure there is enough available storage space before installation starts.
The updated installer, which will only be available if you have not already installed macOS Big Sure 11.2.1, will tell you “There is not enough free space on the selected volume to upgrade the OS,” and it will inform you as to how much additional space is required.