Apple Releases Patch for MacOS Big Sur Bug

Version 11 of the MacOS operating system (OS) for MacIntosh computers, commonly known as Big Sur, is the 17th and most current major release of this OS as of February 2021. It’s the successor to MacOS version 10.15, also known as Catalina. Apple publicly released Big Sur on November 12, 2020, but users began complaining about a bug that could result in serious data loss shortly thereafter. The problem was introduced in MacOS v11.2 and carried over to v11.3, although Apple has since released a patch for this bug.

The installer for the affected versions of Big Sur fails to check if the Mac has the free storage space needed to install this OS. If the installation runs out of space before it completes, the Mac gets stuck in the Boot Recovery Assistant environment. The problem is even worse when the Mac’s data is encrypted with FileVault, since the user can’t even boot up their Mac afterwards. In this case, the user must delete system files or even wipe the entire disk and reinstall a previous version of MacOS. However, the user will lose any data wasn’t already backed up.


Mr. Macintosh performed a rigorous analysis of the bug to determine when the bug will occur. The upgrade to Big Sur displays a pop-up message that displays the free space that it requires. It’s supposed to check the free space on the disk and prevent the user from continuing with the installation if it’s less than what the upgrade requires.

However, the installer doesn’t actually perform this check and will start the upgrade regardless of the space available on the disk. The upgrade will then fail once the disk is completely full, leaving the installer in a repeating loop while it attempts to finish the installation. The user is then unable to access data stored on the Mac.

The Big Sur installer requires 13 GB of storage for itself, and Big Sur OS requires an additional 35.5 GB. The total space that an upgrade to Big Sur requires is thus 48.5 GB. If users have less than 13 GB of space, they won’t even be able to load the installer on the disk. Users with more than 48.5 GB available on their disk will be able to complete the installation. This bug therefore only occurs to users who try to install affected versions of Big Sur when their disk has between 13 and 48.5 GB of available space.


Apple released a fix for the Big Sur bug in late February 2021 in the form of v11.2.1 of the installer, designated as 20D75. It properly checks the disk for available free space, as confirmed by Mr. Macintosh. However, this fix won’t help users who previously attempted an upgrade to Big Sur with a bugged installer, leaving them with a Mac that won’t boot. Apple hasn’t yet provided any official word about a possible solution for this problem.

The lessons that users should learn from this bug is to perform a complete backup before attempting an OS upgrade. In addition, they should ensure their system meets the requirements for the upgrade without relying on the installer to make this check. We do not support Macs, but we do understand that many of our clients use them as personal devices, and as a leading legal IT service provider we keep our clients up to date on ALL topics in IT.


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