macOS Big Sur was officially released to all users in November last year, and since then Apple has released bug fixes and overall improvements. As reported by Mr. Macintosh, Big Sur still has a serious issue that could lead to data loss when users try to upgrade a Mac to the latest version of the operating system without enough space available.
After noticing multiple user reports on the Internet, Mr. Macintosh found that the MacOS Big Sur installer did not check if the Mac’s internal storage had enough space. Because the system starts the update process, the Mac does not respond and data can be permanently damaged.
Apple says that upgrading to MacOS Big Sur for the first time requires at least 35.5 GB of available storage – and that does not include the 13 GB MacOS Big Sur installer. Unfortunately, MacOS will try to install the Big Sur update even if 35.5 GB does not have memory available, and then users may lose all their data.
The update process seems to work perfectly, but users get the following message at the end of the installation:
Failed to prepare the software update.
From that point on, the Mac can no longer boot. Mr. Macintosh was able to confirm that this error affected MacOS Big Sur 11.2 installer and even MacOS Big Sur 11.3 beta installer. At the same time, it does not affect OTA updates from one Big Sur installation to another (such as updating from MacOS 11.1 to MacOS 11.2).
If you have a backup of your data, you can simply delete the entire disk and reinstall MacOS. However, it can be very difficult to recover data without a backup.
If FileVault is enabled, you will need to connect your Mac to another Mac via target disk mode to recover your files. If FileVault is not enabled on your Mac, you can try deleting some files using the Terminal app in MacOS Recovery, which will allow MacOS to successfully complete the update process.
Apple has so far not commented on this bug, but hopefully it will be fixed with the final release of macOS Big Sur 11.3. You can find more information about this issue on the Mr. Macintosh blog.
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