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Mac admin'ing

T2 Macs don’t count as external boot media

I don’t see this documented anywhere in an officially explicit way, so I’m writing a blog post on this. With the introduction of T2-chip Macs, Apple introduced something called Startup Security Utility, and the official line there is that you can change the setting to “Allow booting from external media,” and then you can boot from “an external hard drive, thumb drive, or other external media.”

What the official knowledgebase article doesn’t tell you is that (again, just based on my own testing—if someone has official Apple documentation laying this out in a clear way, please comment below, and I’ll amend this post with a link to the official explanation) is that a T2-chip Mac does not count as “an external hard drive, thumb drive, or other external media.”

So, here’s what I’ve found based on my testing, if you change the Startup Security Utility settings to be as lax as possible:

  • You can target disk mode boot a T1 Mac to a T2 Mac.
  • You can target disk mode boot a T1 Mac to another T1 Mac.
  • You probably cannot target disk boot a T2 Mac to a T1 Mac… I’ve tried this with several machines of varying models, and sometimes it works; other times it doesn’t. I would probably err on the side of assuming it won’t work.
  • You cannot target disk mode boot a T2 Mac to another Mac.

And by “target disk mode boot,” I mean actually booting one Mac from another Mac, not mounting another Mac as an external drive inside an already booted Mac.

What’s been your experience? Are there tricks to get this to work that I’m not seeing?

If you know this to be true, is there official Apple documentation that lays this out clearly?