The macOS Big Sur upgrade is not checking to make sure enough free HD space is available. The upgrade will start, then run out of space and fail. If FV2 encryption is enabled, you will be locked out of your data!
UPDATED 10/07/21 – Latest Updates!!! – For the latest information be sure to check the UPDATES section below.
Quick Summary – When you start the macOS Big Sur upgrade, the installer should first check to make sure your Mac has enough free space available. If the installer finds that you do not have enough free space for the upgrade, it will stop and not let you continue. You should see a pop up message showing you how much space is needed before you can attempt the upgrade again. This free space check is NOT working. The upgrade will start even if you only have 1% of free space left and will fail. Your hard drive is now 100% full and the installer is now stuck in a boot loop attempting to finish the install. This leaves you unable access your data! I will go over all the details below and show you a fix at the end.
I’ve been trying to get a handle on this issue for over a month now. The first signs that this is becoming a larger issue came after the macOS Big Sur 11.2 update was made available. Reports started to roll in saying that the Big Sur Upgrade failed and the only way to fix it was to erase and reinstall macOS. I noticed an increase in traffic to this article. >
https://mrmacintosh.com/reinstalling-big-sur-on-apple-silicon-macs-with-11-0-20a2411-error/
The reason behind the traffic increase was because the M1 personalization error message was very close to the new error.
Big Sur Upgrade out of space error = An error occurred preparing the software update.
Big Sur Upgrade out of space error = An error occurred applying the software update.
Apple Silicon M1 personalization error = An error occurred preparing the update.
This led me to investigate this issue further.