How to Stop Yourself Bricking Your Mac - Disable Secure Boot
If you’re lucky to have a newer Mac with a T2 chip in it, at some point you’re likely going to reinstall the OS to freshen things up.
If “Allow booting from external media” is disabled (see image below) in Startup Security and you use Internet Recovery which then fails at any point (past the point of changing disk partitions) - when hen you then reboot your Mac, it’ll be bricked.
Internet Recovery can fail for many reasons - poor internet connection, checksum failure or any other number of things you don’t control.
The recovery partition will be external media and it’ll refuse to write to the disk in the machine. As expected, other external drives won’t boot either.
Like me, your Macbook will have to go to for a trip to Apple. The lovely folks at the Genius bar were stumped and the likely remedy at the moment appears to be a logic board replacement.
To prevent this happening to you, I would highly recommend that you do the following unless you have very good reason not to:
To allow your Mac to use an external startup disk:
Open Startup Security Utility.
Select “Allow booting from external media”.
There instructions are present on Apples site linked above.
Security is useful, everyone would agree but the default for this surely has to be off/allowing external media. At no point in having this Mac did I set that (I didn’t even know the option existed).