If there is no hard drive space for the new system, the Mac can get stuck in install mode.
If you want to upgrade from macOS Catalina or older to macOS Big Sur, the new system generally requires 35.5 GB of free space. With the older operating systems (up to Mac OS X El Capitan) – 44.5 GB of free memory. The installer itself is almost 13 GB in size, and the computer also needs this storage space during installation. However, “MrMacintosh”, the blog about Macs and macOS, has discovered a serious bug with the current Big Sur installer. Once the Mac has downloaded the DMG file and the user has started the installation, the program does not check whether there is enough space on the Mac for the fully unpacked operating system. The installation starts and is never finished because the Mac hangs up in the boot process if the free memory ran out.
The user changes from a previous system to Big Sur, the point updates (from macOS 11.1 to macOS 11.2 do not seem to be affected) on the Mac are between 15 and 35.5 GB of free space remained. If less than 13 GB is free on the Mac, the installer will not be downloaded from the Apple server, so the installation cannot take place.
All Macs that are compatible with macOS Big Sur are prone to the error. Anyone who still has their data encrypted in the system with File Vault 2 on a T2 Mac is faced with an even bigger problem: The data can only be saved if you use a second Mac for data recovery. Apple has repeatedly increased the security of its systems with the latest updates: With the T2 Macs, for example, it is not possible to start the system from an external hard drive. Since macOS Catalina, the account password is required before the recovery mode. And here comes the biggest catch: this password is not accepted by the system in recovery mode, even though it is perfectly correct.
The only way out, which “MrMacintosh” found in this case, is to connect the damaged Mac to a second Mac with macOS Sierra or Mojave, to start the hard disk mode there and to start the hard disk of the locked Mac in this mode.
The cable is also important, Thunderbolt 3, USB-C to USB-A or USB-C to USB-C 3.0 are required. If File Vault 2 is deactivated on the Mac, the rescue is a little easier: In recovery mode, the Terminal app can be started and large files can be easily deleted.