We actually needed 35GB available when it came to doing the installation. You’ll see below that even 20GB space isn’t actually sufficient when it comes to installing macOS Big Sur though.
MAC MINI KEEPS FREEZING AFTER UPDATE INSTALL
Possibly there isn’t adequate space available on your Mac (we always recommend that you don’t install if you have less than 20GB of space free as your Mac may struggle with the installation otherwise – in fact with Big Sur weighing in at just over 12GB, and Monterey likely to be just as large, you’ll need all the space you can get!) It is likely that if a similar problem happens in Monterey we will see an update soon after launch. Second Big Sur 11.2.1 update fixes Mac installation issue. Read about Big Sur version 11.2.1 (build 20D75) and how it fixes the problem in this article: Apple issued an update to Big Sur that addressed this particular problem on 15 February 2021. As a result the installation would be unable to complete and all the storage available on the Mac would be filled making the Mac unusable. In 2020 when Big Sur launched some Mac users were unfortunate enough to experience an issue where the installer would fail to check how much space was available before installing the operating system update. In this case the problem seems to affect computers that feature Apple’s M1 chip, but there are also reports of Intel Macs being affected.
Problem with the USB 3.0 ports – mainly affecting hubs, but also some USB peripherals do not work properly after the upgrade. If you encounter the bug you may find that force-quitting your applications or rebooting your Mac fixes it.