I noticed an increase in traffic to this article. 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. The first signs that this is becoming a larger issue came after the macOS Big Sur 11.2 update was made available. I’ve been trying to get a handle on this issue for over a month now. This leaves you unable access your data! I will go over all the details below and show you a fix at the end. Your hard drive is now 100% full and the installer is now stuck in a boot loop attempting to finish the install. The upgrade will start even if you only have 1% of free space left and will fail. You should see a pop up message showing you how much space is needed before you can attempt the upgrade again. If the installer finds that you do not have enough free space for the upgrade, it will stop and not let you continue. 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. UPDATED 10/07/21 – Latest Updates!!! – For the latest information be sure to check the UPDATES section below. If FV2 encryption is enabled, you will be locked out of your data! The upgrade will start, then run out of space and fail. In its place would be 'disk0s2.' It is this indented entry that you want to erase.The macOS Big Sur upgrade is not checking to make sure enough free HD space is available.

You might see that it is displayed on the list as:

To be sure you do this right, please observe that the drive may appear in the list as ghosted because it is unmounted. Use the device reference in this next command. It will appear similar but not the same as this: /dev/disk0s2. In the output, you should find your Fusion drive listed. At the prompt paste these lines individually pressing RETURN at the end of each line. If that doesn't work then quit Disk Utility and open Terminal from the Utilities' menu. After doing that click on the Unmount button in the toolbar to see if the disk disappears from the Desktop. Try repairing the disk in Disk Utility using First Aid.

Volume could not be unmounted imac pro