Apple Fixes a macOS Big Sur Installer Bug That Could Result in Data Loss

Published Feb 16, 2021

A revised Big Sur update now checks to make sure you have enough free storage space available before the installation starts.

Apple has fixed a critical bug in the macOS Big Sur installer which used to throw an error message when attempting to upgrade a Mac without the required storage space available.

A revised macOS Big Sur 11.2.1 update, now available in the Mac App Store, properly checks if you have enough free space before the installation starts, Mr. Macintosh said on Twitter.

macOS Big Sur requires 35 gigabytes of free storage space on your computer's startup disk, excluding the 13-gigabyte installer itself.

This couldn't have come sooner as one of the consequences of the broken installer is potential data loss. For people who haven't developed a habit of regularly backing up their Mac with Time Machine, this pretty basic bug could potentially have terrible consequences.

How to Install the New Update

The new update is available through System Preferences > Software Update on your Mac. If any updates are available, hitting the "Update Now" button will install them. You might be asked to enter your administrator password.

You will not see the update if the revised macOS Big Sur 11.2.1 version is already installed on your computer. The revised installer carries a build number of "20D75". To see the build number of the macOS Big Sur software that's installed on your Mac, click the Apple menu and choose the option labeled "About This Mac".

With the About This Mac window open, click the "Overview" tab. You will see the name of the macOS operating system software on your computer, followed by its version number. To reveal the build number as well, simply click the version number.

You can see which macOS version is the latest in a support document on Apple's website.

Big Sur’s Broken Free Space Check

The issue has recently come to light thanks to Mr. Macintosh, which originally reported the problem. The publication discovered that the Big Sur update would download without a hiccup but refuse to start because "An error occurred preparing the software update."

The issue was chalked up to an error in Big Sur's free space check. Mr. Macintosh says people have been complaining about this since the first public version of macOS Big Sur came out in November 2020. After the publication alerted Apple to the problem, the company has moved swiftly to squish the nasty bug.

"The upgrade will start even if you only have 1% of free space left and will fail," the post explains. And with the startup drive nearly 100 percent full, the installer would get stuck in an endless boot loop attempting to finish the installation procedure.

"This leaves you unable to access your data," Mr. Macintosh cautioned.

The only way to fix it required erasing and reinstalling the macOS software. But with the revised update now live in the Mac App Store, Mac owners can go about their daily lives without worrying whether updating to Big Sur might wipe their startup disk clean.