Apple fixes macOS Big Sur bug that caused irretrievable data loss

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Apple has patched a non-exploitable programming bug in its flagship macOS Big Sur operating system (OS) that could lead to irretrievable data loss.

Usually, before the OS undergoes a major upgrade, it performs a check for how much free space is available. In versions 11.2 and 11.3 of Big Sur, however, this check doesn't work as intended, according to Mr Macintosh, meaning the upgrade will start even if you have 1% of space left.

This means that the upgrade will start anyway and will saturate users’ hard disk space at 100%, with the installer stuck in a boot loop in an attempt to finish the install.

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The problem is exacerbated for Mac devices with the T2 security chip and FileVault 2 encryption enabled. For those with a T2 Mac, they will be unable to get into macOS recovery because their password will not work.

Enabling the hard disk encryption software FileVault locks people out due to a failure to accept their passwords in the normal recovery prompts, the Mac researcher showed.

If FileVault is enabled, users will be prompted to enter their admin password before accessing recover, but it won’t be accepted. If users then try to reset their password with a personal recovery key or AppleID, the reset process will fail. Even Target Disk Mode, which turns the macOS device into an external hard drive for another Mac, will fail.

If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade. If upgrading from an even earlier release, macOS Big Sur requires up to 44.5GB of available storage.

The range of available hard drive space where the bug would kick users into a boot loop was between 13GB and 35.5GB of free space. This is according to a deep-dive video compiled by Mr Machintosh breaking down the issue and how it manifests.

Thankfully, Apple has released a fresh installer, macOS Big Sur 11.2.1, which now checks for free space properly before applying any major upgrade to the system.

The issue isn’t a new one, with users reporting problems installing Big Sur as far back as November, losing their data in the process.

Keumars Afifi-Sabet is a writer and editor that specialises in public sector, cyber security, and cloud computing. He first joined ITPro as a staff writer in April 2018 and eventually became its Features Editor. Although a regular contributor to other tech sites in the past, these days you will find Keumars on LiveScience, where he runs its Technology section.