iMac M1 downgrade to Big Sur

Hi domcaz,

Yes, you can currently downgrade an M1 iMac to macOS Big Sur 11.3 or newer - even if it shipped with a newer macOS. There are two primary ways to do this, and both of them involve an erase.

If you have another Mac running macOS Monterey 12.4 or newer, you can use Apple Configurator on the other Mac to downgrade both the firmware and macOS on the iMac. Download the restore file for your desired macOS version, put the iMac into DFU mode, connect the two Macs, and drag the restore file on top of the DFU icon that appears in Apple Configurator. Choose to restore.

(The IPSW restore file site linked above exclusively uses genuine Apple server links and downloads all restore files straight from Apple. It doesn't act as an intermediary.)

Otherwise, you can prepare a bootable installer for macOS Big Sur, and then use the bootable installer to erase the iMac and install macOS Big Sur. Please note that Apple only provides recent versions of the Big Sur installer, unlike the other option above. Also, some system firmware (and probably Recovery) will remain on newer versions instead of being downgraded.

EDIT: If you're going with the restore option using Apple Configurator, some cables work better than others for macOS Big Sur in particular. I've found the most success using the USB-C to USB-C charge cable that came with my M1 MacBook Air. Apple's 0.8m Thunderbolt 3 cable fails for some reason.

It’s might be worth a shot…then again, if it bricks your Mac, good luck trying to get warranty support. Search the internet if you still wish to proceed, or wait it out for old-timers to pitch in. Good luck!

domcaz wrote:

I need to run some server software that only runs under Big Sur (or older).

If I purchase a brand new iMac M1 today, I presume it will ship with Ventura installed. Since I know the M1 iMac originally shipped with Big Sur - I'm assuming a brand new iMac be downgraded, is that correct? Or has the firmware since been updated to prevent this?

It is true that in the past you could downgrade a Mac to the version of macOS that originally shipped with a particular model, I don't know if that's still the case. If nothing else, you should be able to run Big Sur as a VM. Nothing earlier would run on Apple silicon Macs.

I think the situation is you can't downgrade to a version of macOS shipped with the device.

You can try and downgrade to Big Sur but why? On 9/26/2023, Sonoma will be released and Big Sur will be relegated to status of old and unsupported.

Apple apparently supports the three latest versions of macOS. Big Sur will fall to fourth on the list as of 9/26

Ventura is a good OS. But it too is facing old age. Not like BS, but it's getting there.

domcaz wrote:

Thanks - yeah I know I can Parallels if it comes to it, but I'm just trying to ascertain if a downgrade is possible. Not much info on this on the internet that I can find!

You're welcome. Unless someone else has done it, the only way to know would be to purchase the M1 iMac then try to download Big Sur. If it says not available, you can't. If it says do you really want to download an older version of macOS, you can.

rkaufmann87 wrote:

Because you cannot install an earlier version of Mac OS than what the computer shipped with. Best to use an app that is more up to date than what you are attempting.

This was the golden rule with Intel-based Macs, but Apple changed a lot with Apple silicon. It seems the new rule for now is you can downgrade to the first version of macOS that supported the product model, if Apple accepts it.

Apple silicon Macs have DFU mode burnt into the chip's ROM, which means you can reach it even if the firmware is corrupted. When Apple Configurator tries to restore a Mac, it grabs the latest restore file from Apple and tries to apply it. The restore file contains everything needed, but the process requires phoning home to Apple with the target Mac's info and info about the restore file. If Apple approves the restore, it signs the provided info for that specific Mac.

So far, Apple's servers are still approving restore requests for all compatible versions of macOS - including macOS Big Sur 11.0.1 for the M1 MBA, MBP, and Mac mini. It will be a sad day if/when Apple decides to only allow using the latest restore files, just like it currently does with iOS and iPadOS.

Not possible, unfortunately.

Because you cannot install an earlier version of Mac OS than what the computer shipped with. Best to use an app that is more up to date than what you are attempting.

Whatever you do I suggest that you download Big Sur while it's still available and make a USB loader for it, just in case you need to install it in the future.