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I'm looking at buying a new 21.5" mid 2017 iMac which will ship with macOS Mojave 10.14.

I have some software that will only run on macOS Sierra 10.12.

Is it possible to downgrade to macOS Sierra 10.12 on this iMac by wiping the hard disk and reinstalling the OS from a USB installer? Does the mid 2017 iMac have the drivers to run macOS Sierra? If not do they have the drivers to run macOS High Sierra?

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  • It might be easier to install Sierra/High Sierra into a VM and run the legacy software there. – nohillside♦ Nov 28 '18 at 16:10
  • @nohillside interesting approach.. what would the pros and cons of that be ? I currently run 10.11 and 10.12 on my other machines with no other issues, so ive never had the "need" to have access to 10.13 > – sam Nov 28 '18 at 16:31
  • You may want to benefit from the feature set of Mojave in general, or from the improved integration with iOS devices, or ... – nohillside♦ Nov 28 '18 at 19:51
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The mid 2017 21.5" iMac (both Retina 4K and non-retina models) originally shipped with macOS Sierra 10.12.4. Thus, both macOS Sierra and macOS High Sierra have the drivers to run on either one of the iMac's.

It would be possible to run the last (publicly available) release of both macOS Sierra 10.12.6 and macOS High Sierra 10.13.6 on either one of the iMac's by installing via USB installer or via Internet recovery.

The installer can use downloaded from the Mac App Store:

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    Thanks, i actually have a copy of the installers saved on my drive, as i always download them for safe keeping regardless if im doing the upgrade. From past experience although the mac app store web link will work, when you try to download the file via the mac app store it will fail saying the file is no longer available. – sam Nov 28 '18 at 15:44
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    @sam That's good. At times I too though of adopting the practise of keeping OS installer backups, but have not done yet. I hope Apple doesn't start the practise of signing macOS releases just like they do for iOS. – Nimesh Neema Nov 28 '18 at 15:46
  • apple signing macOS releases dose seem like the logical next step for them in locking down their eco system. I really hope they dont though as in a business users context the inflexibility would be unbearable. – sam Nov 28 '18 at 16:33

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