I don't think this question has been asked before - as much as I don't want to upgrade to a new M1 MacBook Air, my 2015 MBA is nearing the end of its life, despite my many attempts to keep it going. The keyboard is giving me fits - ghost typing, then not being responsive, typing the letter "r" repeatedly in search engines and taking over my mouse and not letting me select items, etcrrr and it's just not worth trying to repair it. I've tried resetting SMC and all those things, checked the cables and their connections. Blew compressed air under the key. Even tried a bluetooth keyboard, and it's still doing those things. It's had a good life. I recently upgraded the internal SSD to a NVMe drive thanks to the forum, and learned how to test drive the newer macOS on external drives, but I primarily use Mojave, so my question is if I take out the SSD with Mojave and put it in an enclosure, can I boot and use it on the new M1? That would be my preference as a short-term workaround since I am also in my work's busy season and also in the middle of a move. If I can't do that, I'm going to have to rethink my options.
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Newbie Question - using old macOS on external SSD
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What Mark said - The reason is that prior versions of macOS will not come with the hardware support drivers for the newer machines. Sometimes they’re similar enough that you can just copy over a kexts from a newer release and it’ll work, but the M1 is a whole other CPU architecture and Mojave is an x86_64 OS. It wouldn’t be able to run on an ARM instruction set chip at all with the builds that’ve been released
Thanks for the quick response - I understand that the M1 will have Big Sur installed, but can I hold the option key and boot to an external drive that has Mojave?
No; See my earlier comment for explanation why. - You can boot external drives, but Mojave does not support the M1 hardware