A guide to what you can do when a Big Sur or other macOS update goes wrong, from Safe mode to DFU mode and Configurator 2.
Safe mode
From Startup Manager to Internet Recovery on Intel Macs, and how to engage each of those in an M1 Mac. With comprehensive diagrams.
M1 Macs don’t use that warren of startup key combinations, but a logical structure of choices, mostly when starting up in Recovery Mode. Here are the full details.
The Mac is now in a documentation crisis. Documentation debt is leaving many Mac users floundering around in the dark.
Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Disk integrity checks appear to have been dropped from Safe Boot. OpenDirectory databases are rebuilt. With a full listing of all blocked extensions.
Start up in Safe Mode: should it start by checking and repairing your boot disk, or should you do that separately in Recovery Mode?
When should you reset your SMC and NVRAM, repair permissions, install a Combo update, or re-install the whole of macOS?
Safe mode, or Recovery? Which type of Recovery? How to engage all those useful modes at startup, and when shutting down.
Start up in Safe mode, and it’s stable; restart in ordinary mode, and your Mac is in trouble. How to identify and fix the problem.
The Eclectic Light Company 