Downgrading to Big Sur

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Downgrading to Big Sur

I've got 5 macs here, all on Monterey. There is one machine, a Mac Mini 2018 which I would like to downgrade to Big Sur (I'll spare you why). The issue is that no matter what I do, this machine will not allow me to do the downgrade. Data on the drive is completely expendable so I've formatted it. It's got nothing now. Here's what I've tried.

I downloaded Big Sur from the Apple Store onto my 2019 iMac (a few days ago, before the Mac App Store was having difficulties).
I followed the instructions to make a bootable USB drive.
I go to the Mini and hold option, boot into the image from the USB drive, when I click "Install Big Sur", it comes back with the big sur installer is damaged.

Online searches say something about an expired certificate which makes zero sense because I literally just downloaded the thing. But anyway, I used the softwareupdate terminal command to retrieve a fresh copy, rebuild the usb drive, same thing.

Then I tried a different USB drive. Same thing.

Then I re-downloaded (third time) the image using installinstallmacos.py. Same thing.

I checked if my clock was off (terminal: date). Nope, it's correct.

Then I booted into Internet Recovery thinking since the machine shipped with Mojave, it might present that as an option. Nope. It wants to install Monterey.

I also tried just launching the install Big Sur app to install onto a different SSD drive and it won't launch from Monterey, but I have no macs that aren't Monterey now.

I'm completely bald but if I had hair, I'd pull it all out. I've been working on this all day. The simplest task of installing an operating system one level back on a fresh disk is seemingly impossible yet online guide after online guide seem to make it seem easy - even Apple's support docs make it sound possible.

I think if I could get past the "installer is damaged" error, it would work. Is that happening because it's reaching out to Apple and Apple is having issues right now? That's all I can think of. I'm out of ideas.

Mac mini, macOS 12.0

Posted on Dec 11, 2021 5:27 PM

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Answer 1

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Sean Lemson Author

Level 1

40 points

Dec 14, 2021 7:15 PM in response to Sean Lemson

It appears to be working. Unfortunately, I changed two variables at the same time (I'm a terrible scientist) so I don't know which made the difference.

The first thing I changed is that I used installinstallmacos.py to download an earlier version of Big Sur. I had been downloading 11.6.2. Instead, I downloaded 11.5.2

The second thing I changed is that I switched from a USB Stick to an SSD drive I had around.

The combination of one or both of those things got me past the error message about a damaged installer.

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Answer 2

PRP_53

Level 10

92,431 points

Dec 12, 2021 3:40 AM in response to Sean Lemson

To perform this action will require booting from a Bootable Installer.

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer

Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive.

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map.

6 - Once that is done >> backup out of Disk Utilities and choose install macOS.

7 - Follow the prompts and it may automatically reboot several time.

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

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Answer 3

Barney-15E

Level 10

117,142 points

Dec 11, 2021 5:35 PM in response to Sean Lemson

You can’t install over the top of a newer OS. While booted into the USB installer, use Disk Utility to completely erase the drive.

Use the View pop up menu to select Show All Devices. Select the drive device and Erase.

Now, I don’t know if the USB is really damaged, but I know you cannot replace a newer OS with an older without completely erasing the drive.

Another thing you can do if you have the space is add a Volume to the Monterey drive and install onto that using the USB installer. Then you could boot into either.

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Answer 4

Sean Lemson Author

Level 1

40 points

Dec 14, 2021 5:05 PM in response to PRP_53

I followed all these steps. I actually had already formatted the hard drive the way you suggested. It's completely blank (no restore partition). I get to Step 6 and get the same message I always get:

"This copy of install macOS Big Sur application is damaged, and can't be used to install macOS."

I'm not sure what you mean by "qualifying computer" but I built this image on the USB thumb drive on my iMac Retina 5K, 27-inch 2019. Processor is a 3.6 GHz 8-core intel core i9. The Mac Mini is also an intel chip. This iMac and the MacMini both ran Big Sur for a long time so I'm assuming that's what you mean by qualifying. If so, it is qualifying.

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Answer 5

Sean Lemson Author

Level 1

40 points

Dec 14, 2021 5:09 PM in response to Barney-15E

Another thing you can do if you have the space is add a Volume to the Monterey drive and install onto that using the USB installer. Then you could boot into either.

I don't know how I would do this as I literally cannot launch the Big Sur installer FROM Monterey. It never even gives me the option to choose a volume to install it. It won't launch.

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Answer 6

Barney-15E

Level 10

117,142 points

Dec 14, 2021 7:22 PM in response to Sean Lemson

Another thing you can do if you have the space is add a Volume to the Monterey drive and install onto that using the USB installer. Then you could boot into either.

I don't know how I would do this as I literally cannot launch the Big Sur installer FROM Monterey. It never even gives me the option to choose a volume to install it. It won't launch.

As I stated, you must do that from the USB Installer, not from Monterey.

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Downgrading to Big Sur

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