Thread: running Catalina and High Sierra on same Mac
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Old 07-05-2020, 07:44 AM   #37
pwalker8
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Quote:
Originally Posted by helpplease View Post
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.
As a rule of thumb you can't run a machine on a IOS version prior to when the first model of that machine was shipped. Mostly, it's a situation when the older IOS version doesn't have the device drivers needed for the newer hardware. Thus I can boot my old mac mini and iMac on an external drive with Sierra on it, but not my 2018 macbook pro. I tried the same thing that you did (but with Sierra) and got the same error.

I understand there are hacks you can do to force it to try, but I suspect you may run into so odd issues. For me, it wasn't worth the effort. I can book my iMac on an external drive with Sierra, which suits my needs. If you really need to run High Sierra, then you might consider getting an older machine. I'm not sure what incompatibilities you are running into.
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