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macOS Big Sur upgrade reportedly causing data loss if low on drive space

macOS Big Sur
(Image credit: guteksk7 / Shutterstock / Apple)

Mac owners who are upgrading to macOS Big Sur should be very careful if they are running low on drive space, as in this case, the process could reportedly lead to data loss.

The issue here is that an initial upgrade to Big Sur requires a minimum of 35.5GB to be available on the drive, plus 13GB for the macOS Big Sur installer itself (making 48.5GB in total). If you don’t have that much space, then things can go badly wrong, as reported by Mr Macintosh.

The problem is that apparently macOS doesn’t check if you have that amount of drive space before starting the installation process, and when the installer subsequently runs out of room, some folks are reporting getting an “error occurred preparing the software update” message.

The Mac then becomes stuck in the Boot Recovery Assistant environment, with no way to recover. Well, no easy or obvious way to recover, anyway, leaving the data on the disk in danger of being lost – more on that in a moment.

Note that this isn’t affecting those upgrading to a different point release of macOS Big Sur – say, from 11.1 to version 11.2 – but those using the full installers to upgrade their machines straight to either Big Sur 11.2, or the beta of Big Sur 11.3.

Sticky situation

The possible recovery procedure depends on the exact scenario you’re in, as explained in the blog post by Mr Macintosh, but if you have FileVault encryption enabled on your system, things can get very sticky.

Indeed, in the latter case, the only way to go about righting your system and recovering your data is by using a second Mac (running macOS High Sierra or Mojave) and hooking the two machines up. So, many folks will be in trouble there, as it isn’t exactly common to have more than one Mac (or easy to borrow one, if you don’t).

The easiest course of action for some folks may simply be to start from scratch, choosing to erase and reinstall macOS; providing that they have an up-to-date backup, and won’t lose data that way.

Apple hasn’t acknowledged the bug, and as mentioned it is still present in the beta of Big Sur 11.3 – but hopefully now it’s been highlighted in a major way, the Mac maker will be looking into the issue (if it isn’t already under investigation). We’ve contacted Apple for a comment on this matter, and will update this story if we hear back. 

As you may have seen, macOS Big Sur 11.2 was only recently deployed, delivering some important fixes including a solution for the Bluetooth problems which have been plaguing some M1-powered Mac owners.

Via 9 to 5 Mac