System Updates vs Firmware Updates That Disable Downgrade Compatibility

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We have a MacBook Pro 2018, that originally shipped with HighSierra, but purchased with a later Mac OS : Sonoma.
I noticed there is no downgradeability or backward compatibility to High Sierra( or up to Ventura)

Question: might there be a list of Mac OS system updates that refer to or spec firmware or other system updates that include disabling backward compatibility?
For instance, even after erasing the drive in Disk Utility, this system’s bare minimum is
now Sonoma. Also the same using Recovery mode.

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Your system firmware and T2 iBoot firmware can not be downgraded. But you shouldn’t have to. It is supposed to be backward compatible with older OS releases for the platform. Of course, when there’s a critical bug, this can become a catastrophe (go read some other threads here about my experience with iBoot firmware and macOS 14.7), but there’s pretty much nothing you can do about it.

You can reinstall system firmware using another Mac with USB-C and the Configurator app, but you can only install current versions. Like old versions of iOS, you can’t install images that Apple no longer signs, and they generally stop signing these images one week after new ones ship.

But that’s firmware. macOS is a different story.

According to Apple, you have three options for reinstallation from Recovery mode:

  • Hold ⌘ R (command-R) at power-on to boot into a Recovery mode that will let you reinstall the most-recently-installed version of macOS
  • Hold ⌥⌘ R (option-command-R) at power-on to boot into a Recovery mode that “might” offer the latest macOS compatible with your Mac. I’m not sure what “might” means here, I assume that the truly latest version isn’t always available via this mechanism.
  • Hold ⇧⌥⌘ R (shift-option-command-R) at power-on to boot into a Recovery mode that “might” offer the macOS that originally shipped with your Mac, or the closest version still available.

If none of those options are desirable or do what you want, you can also go this Apple support page: How to download and install macOS - Apple Support and use one of the provided links to download installers for the latest version of each major macOS release.

With one of these standalone installers, you can run it as an app to upgrade your macOS. You can also create a bootable installer from one of these. Note that they are big - recent releases of macOS will require a 32GB storage device. I did it most recently using a micro-SD card and a USB adapter, but any USB storage device will probably work.

If you make and want to use a bootable installer, you should power off. Then connect the installer device and power on while holding down the ⌥ (option or alt) key. You will be presented with a list of bootable volumes. Pick the one with the installer. It will boot to a Recovery mode that will install that version of macOS.

So far, for all of the above approaches, you can only upgdade macOS on a running system. If you want to downgrade, then what you must do is:

  • Make a full backup of your Data volume (Time Machine will work, as will various cloning utilities), because the next steps will erase everything.
  • Boot to Recovery mode with the installer you want to use.
  • Use Disk Utility from there to erase the existing macOS volumes - system and data.
  • Install macOS.
  • Use Migration Assistant to restore everything from your backup.

If the version you’re downgrading to is close to the one you came from (e.g. when I downgraded from 14.7 to 14.6.1), you might be able to just delete the system volume and let the macOS installer re-create it and re-link it to your data volume. This mostly worked for me (I had to jump through a few hoops to get the Apache web server working afterward), but it is not a supported behavior and could create subtle problems, so I don’t recommend it.

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One more followup.

The links from Apple’s installer-download page only provide the latest version of each major release.

If you want an installer for a specific version that is not latest (e.g. if you want to install 14.6), there are sites that link to the images from Apple’s CDN.

One popular site for this is Mr. Macintosh. Useful links from that site include:

Additionally, all of what I wrote in my prior messages is for Intel Macs. The procedure for Apple Silicon is different. Since I have never tried it (I don’t have an Apple Silicon Mac), all I know is from articles written by others. That having been said, here’s a good link:

A quick summary of that article

  • Apple Silicon does not support Internet Recovery

  • The way to boot into Recovery mode is different

  • There is an additional “System Recovery” mode, which can be used if your normal recovery mode doesn’t exist. It can reinstall macOS and macOS Recovery

  • You can still boot from a USB-based bootable installer storage device.

  • As a last resort, you can power it on to DFU mode and use Configurator from another (suitably recent) Mac to reinstall everything, including the hidden system partitions on the internal SSD.

    Reinstalling with Configurator uses an “ipsw” file instead of a normal macOS installer. It will by default download one from an Apple server. I think you can also download one yourself and install it (e.g. to install a specific version), but I don’t know this for sure.

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That is not really correct as only the name has changed. You need an internet connection for basic recovery. From Apple’s page:

“Reinstalling macOS requires an internet connection. To connect to the internet, you can use a wireless or wired network connection. If you’re trying to connect to Wi-Fi through a captive portal (for example, at a coffee shop, library, or hotel) or an enterprise network, you might not be able to access the internet in macOS Recovery.”

Even if you have downloaded the “full” OS installer, it still downloads additional files from the internet.

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You (and the Apple link you cite) are describing something different.

Internet Recovery on an Intel Mac is a mode in the computer’s BootROM code that will download and boot a RecoveryOS instance, even if your local storage is completely wiped.

This doesn’t exist on Apple Silicon. Recovery mode and System Recovery mode both boot a RecoveryOS from your SSD. And booting a USB installer uses pre-boot firmware from your SSD in order to install RecoveryOS from that installer. But if your SSD gets completely wiped, you need to use Configurator to install anything.

This is completely independent of the fact that macOS installers phone home to authenticate themselves, download firmware updates and (sometimes) download the rest of the installer package - which happens on both Intel and Apple Silicon Macs.

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While you’re giving more detail, the fact is regardless of Intel or AS, you still need the internet and when you quote from the article that Internet Recovery is not supported, it implies something different and can confuse those who are having difficulties with their system. Both systems can get the user back to a bootable OS but need the internet to do that.

Apple Configurator is also needed for firmware issues with Apple Silicon.

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Again, you’re misunderstanding the term.

“Internet Recovery” is not a generic catch-all term meaning you need Internet access in order to reinstall your system.

It is a very very specific feature that Apple used to advertise with that specific phrase and has never referred to anything else.

In the PowerPC and earlier days, Apple would bundle recovery and/or OS installation media (CDs, DVDs or floppies, depending on the age of the computer) with hardware. But when Apple stopped including optical drives in Macs (first was the Mac Book Air), they needed an alternative. Those first-generation Airs shipped with a USB flash (or maybe ROM) drive containing a bootable installer.

But very soon afterward, Apple stopped shipping recovery media altogether. Instead, they added two recovery options that would not require external media.

The first was “Recovery Mode”. Power-on with a special key combination held down and your Mac will boot into a special mode that can download and install macOS. This booted RecoveryOS (as it would later be named) from a hidden partition on your Mac’s internal storage.

But what if your internal storage got erased? Or if you upgraded it, as was possible at the time? For that, there was “Internet Recovery”. Power-on with a different special key combination held down and your Mac will boot from code in ROM. Just enough to connect to the Internet, then download and install RecoveryOS on your internal storage device. Then it would boot into Recovery mode from what it just installed.

This is the only feature that Apple has ever called “Internet Recovery”. It is present on nearly all Intel Macs and was never present on anything with Apple Silicon, which relies on Configurator for recovery if your storage device doesn’t have a valid RecoveryOS installation.

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The history of OS installations is irrelevant as way back then, you could install the OS without needing an internet connection but now, it’s different as I know from a recent issue with my M1 Air.

There is no misunderstanding on my part regardless of what the process was/is called: to get the OS on AS, you still need an internet connection for the Recovery process unless you already have it on a flash drive:

" * You need a broadband internet connection that uses DHCP over Wi-Fi or Ethernet. Some types of network connections can prevent installation from macOS Recovery, including those that use a required captive-network login, 802.1X security, or PPPoE (unless a router is handling the PPPoE connection).

Published Date: September 30, 2024"

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Hello,
Thanks for the comprehensive stuff. I am going to save this for future reference.
While I have lost track of all the details of my attempts … mainly I wanted to have a
separate volume with High Sierra or even Big sure on the computer in question so some older apps could be run.
I also notice that there were various anomalies in how this computer would boot as a target volume.
In the connections that would recognize the computer in target mode, nothing other than Sonoma could be installed.
On some other attempts with different computers, the computer would not even mount as a target volume
Bottom line, the Sonoma upgrade to this particular computer basically change things in one direction … onwards and upwards I guess …
I did notice to my surprise that the system does have one more upgrade compatibility … to Sequoia which will extend the usefulness of this system at least.
Thanks

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