A very important bug has just been detected by several users who have had problems updating their Mac to macOS Big Sur. It turns out that when starting the update process, the system does not check if there is enough free space on the hard disk to do this process.
From there you can already imagine how brown it may be that in the middle of the installation the Mac runs out of available storage, and of course, now "neither forward, nor backward". So while Apple doesn't fix it, make sure you have a free space on your Mac's hard drive before updating to macOS Big Sur first.
Big Sur macOS was officially released to all users in November last year, and since then there have been several logical updates to bug fixes tweaks and general improvements. However, Big Sur still has a serious problem that can lead to data loss when users try to upgrade a Mac to macOS Big Sur without enough available storage.
After noticing several complaints from users on your website, Mr. Macintosh found that the macOS Big Sur installer does not check if the internal storage of the Mac has enough free space to perform the update. As the system begins the update process, your Mac It stays blocked and the data it houses can be permanently damaged.
Apple says upgrading to macOS Big Sur for the first time requires at least 35,5 GB of available storage, and this does not include the macOS Big Sur 13GB installer. Unfortunately, even if your Mac doesn't have 35,5GB of available storage, macOS will try to install the Big Sur update, and that's when the problem comes.
If you update your existing macOS Big Sur to a new version, no problem
If there is not enough space, the update process seems to work perfectly, but when the installation is finished, the message 'A error when preparing the software update. '
From that moment on, the Mac no longer starts. Mr. Macintosh confirms that this bug affects the macOS Big Sur 11.2 installer and even the macOS Big Sur 11.3 beta installer. At the same time, this does not affect OTA updates from a Big Sur installation to a new version (such as upgrading from macOS 11.1 to macOS 11.2). The crash only happens when you install macOS Big Sur for the first time.
If you have a Backup of your data, you can simply erase the entire disk and reinstall macOS. However, recovering data without a backup can be very difficult.
With FileVault enabled, you have to connect your Mac to another Mac via target disk mode to get your files back. If FileVault was not enabled on your Mac, you can try deleting some files using the Terminal application in macOS Recovery, which will allow macOS to complete the update process successfully.
Apple hasn't commented on this bug so far, but presumably it will quickly fix it with the final release of the new macOS Big Sur 11.3 version.