Friday, Jun 4, 2021 [Tweets] [Favorites]

Downgrading BridgeOS

Mr. Macintosh (tweet):

Let’s go over a quick example of why you might want to downgrade BridgeOS. You updated to macOS Big Sur 11.4 from 11.3. After the update, you’ve found that something is not working right. The T2 chip handles many things including, fan speed, battery, power, charging & sound (speakers & microphone). So in this example, maybe you are getting audio cracking noise (has happened in the past). Now you thinking that the new version of bridgeOS might be causing problems on your T2 Mac.

The 2nd example is macOS Update testing. If you are a system administrator in charge of updating a large fleet of Mac testing is very important. Part of that macOS Update process is updating BridgeOS. An example of this is if you upgrade a T2 Mac from 11.3 to 11.4. BridgeOS is updated in the process. Now that this T2 Mac is on the latest version of BridgeOS it normally can not be downgraded. Even if you boot back to recovery and install 11.3, BridgeOS will remain the same updated version. Set this Mac on the shelf because you will never be able to take it through a full update process again until 11.5.

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You could always upgrade BridgeOS via automatic download with Apple Configurator 2.

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We can now download full BridgeOS IPSW Files directly from Apple the same way we do now with Apple Silicon M1 Macs. We can then use the BridgeOS IPSW File to restore/revive BridgeOS to your T2 Mac. The difference here is that Apple WILL stop signing for previous versions of BridgeOS. The signing process follows iOS and is canceled usually about one week after the release of a new update. Apple leaves ONE previous version signed (for 7 days) so you can now downgrade to that version!

Previously:

Update (2021-06-13): Howard Oakley:

Let me ask you a simple question: supposing you installed the Monterey beta on an external disk, what would happen to that Mac’s firmware and its Recovery features? Given that Monterey is likely to bring firmware updates to most if not all Macs, how might that affect yours? That’s what I try to answer in this article – and it’s of great importance to all those who install beta-releases, as well as everyone considering upgrading in the autumn/fall.

The answer to these questions depends on which architecture your Mac has, and how it stores and maintains the different parts of what we loosely refer to as firmware.

1 Comment

One of the most consistently annoying things about Apple for the last decade is their unwillingness to let people downgrade. I get not wanting to have unsecured systems out in the world, but there are so many legitimate reasons to need to downgrade that it shouldn't matter. Let people do what they want with the devices they bloody well own.

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