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You can, but the older version of macOS has to be new enough to support the hardware you're running, and it has to be compatible enough to allow booting. In ge

You can, but the older version of macOS has to be new enough to support the hardware you're running, and it has to be compatible enough to allow booting. In general, the oldest macOS you can boot on any given Mac is "the version the Mac shipped from the factory with". So, for example, the 2017 iMac Pro can't boot a 2016 or earlier macOS. This is still unsupported, mind you — if your 2017 iMac Pro is firmware updated to 2020 and you're booting a macOS from 2018 on it, you may encounter random bugs and/or crashes and/or mysterious issues due to assumptions made by the older macOS that are no longer entirely valid on 2020 firmware — but in general, it should work as long as the macOS is newer than the hardware. (You probably won't be able to boot an M1 macOS external drive on an Intel Mac, and vice versa, for other reasons unrelated to this. In that scenario, Migration Assistant is probably your only choice anyways.)

Wowfunhappy 6y ago
I use Mavericks because I like Mavericks, and a few months ago I discovered something bizarre: the mere act of booting a Mavericks installation USB on my 2014 Macbook Air appears to wipe the hard drive. As in, I'm not running the installer and I'm not opening Disk Utility, I'm merely telling the computer to to boot from the USB key. It seems that by the time I've reached the language select screen, the internal hard drive's filesystem is toast. I did this several times because I couldn't believe what I was seeing. I can't say for sure where the source of the problem lies—maybe something is weird with my installation media—but my theory is that a firmware and/or microcode update subtly broke something in Apple's old installers, possibly when they added APFS support. I looked into downgrading the firmware, but it seems to be impossible from what I can tell[1]. Mind you, outside of this one quirk the OS and installer run fine. --- 1: https://forums.macrumors.com/threads/guide-how-to-get-back-old-efi-after-downgrade-from-yosemite.1803934/post-28311855 https://forums.macrumors.com/threads/guide-how-to-get-back-o...
- floatingatoll 6y ago
  Good to know, I hope you didn't lose any non-backed-up data!
  - Wowfunhappy 6y ago
    Oh, yes, nothing important! Not only do I keep lots of backups, but the laptop is very much a secondary machine to my desktop, especially in the age of COVID. All I lost was the few hours it takes to reinstall my apps and such. :)

Wowfunhappy 6y ago

I use Mavericks because I like Mavericks, and a few months ago I discovered something bizarre: the mere act of booting a Mavericks installation USB on my 2014 Macbook Air appears to wipe the hard drive. As in, I'm not running the installer and I'm not opening Disk Utility, I'm merely telling the computer to to boot from the USB key. It seems that by the time I've reached the language select screen, the internal hard drive's filesystem is toast. I did this several times because I couldn't believe what I was seeing. I can't say for sure where the source of the problem lies—maybe something is weird with my installation media—but my theory is that a firmware and/or microcode update subtly broke something in Apple's old installers, possibly when they added APFS support. I looked into downgrading the firmware, but it seems to be impossible from what I can tell[1]. Mind you, outside of this one quirk the OS and installer run fine. --- 1: https://forums.macrumors.com/threads/guide-how-to-get-back-old-efi-after-downgrade-from-yosemite.1803934/post-28311855 https://forums.macrumors.com/threads/guide-how-to-get-back-o...

floatingatoll 6y ago
Good to know, I hope you didn't lose any non-backed-up data!
- Wowfunhappy 6y ago
  Oh, yes, nothing important! Not only do I keep lots of backups, but the laptop is very much a secondary machine to my desktop, especially in the age of COVID. All I lost was the few hours it takes to reinstall my apps and such. :)