I need to erase a failed Big Sur update and reinstall on my iMac 2017. Disk Utility won’t allow me to erase the top-level Fusion Drive but at the next level is Container disk2, and within that are two volumes: Macintosh HD - Data, and Update. Should I erase the entire Container, or just one of those two folders?
There’s a happy ending, or fairly happy. Someone on the Apple Communities forum (discussions.apple.com) told me to erase the Container. I tried to do that but the erase failed. So then I erased the Update folder, which worked.
I then installed Big Sur from a bootable backup HD, and the only option offered as a place to install it was the Update folder. This time the installation worked. The folder structure is now extremely weird but I guess I can sort that out eventually…