[helpplease]

Hi,
@odamizu
I tried to boot off my old SDD harddrive in an external caddie (running high Sierra 10.13.6) from new MacBook Air (shipped with Catalina), and it refused to allow it. Refused even after removing secure security boot feature of Catalina (eg recovery mode, utilities/security settings and change allow boot external, and change no security).

On boot up, startup disk set to my old harddrive running high Sierra, gave me white circle with line thru it. Googled that "white circle with line thru it" and said it's forbidden to downgrade from shipped version.

I'm thinking, really? This must be possible some how. I have a full working external harddrive (SSD) just my MacBook Pro motherboard died, so I have a new MacBook Air. I don't want Catalina as presents all sorts of unplanned problems with applications and ebooks.

Any ideas?

I am up to trying what this thread is suggesting dual boot. But if dual boot works, then it should work from an external. Or maybe dual boot only works from older MacBooks (pre Catalina ) where the dual boot includes high Sierra. So you can dual boot for OS X shipped with and later, but not earlier. I don't know.

I hope it makes some sense.