APPLE INSIDER
An issue with Apple’s macOS Big Sur installer allows users to upgrade from an earlier operating system without first verifying that the target Mac has enough free hard drive space, resulting in installation failure and data loss.
The problem is present in currently shipping Big Sur installers and appears to date back to the first macOS 11 builds launched in November, reports Mr. Macintosh. Interestingly, subsequent point releases and delta updates are unaffected by the bug.Apple’s installers fail to check system hard drive space on initiation and continue to run through the installation process until all storage is exhausted. The resulting failure can lead to an install loop, purgatory in Boot Recovery Assistant with a pop-up reading, “An error occurred preparing the software update,” or display of Big Sur’s Recovery startup screen that shows no startup disk available.
Source: APPLE INSIDER