I have 11.6.5 currently installed and want to go back to a Time Machine copy when it was 10.12.6, but it's not able to. But if I try to reinstall 10.12.6 from a .dmg, "This copy of "Install macOS" application is too old to be opened on this version of macOS"
Is there *any* way to go back short of wiping the whole thing and starting over from an install disk? (The new version bolloxed up some apps, which I probably can't get again if I wipe it)
This method will WIPE ALL Data and there is No Recovery - Period.
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.
Only as Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.
Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.
Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.
Once that is done read on for preparing the Destination computer >> Only works on Intel Based Apple Computers.
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. This applies to macOS 10.14 Mojave and above.
5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled 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.
You could install to an external drive, the problem is that 10.12.x cannot be installed on an APFS partition/drive as is required by 11.6.5
How to get old versions of macOS - Apple Support
Then make a bootable USB installer...
How to create a bootable installer for macOS - Apple Support
I think you didn't format the whole drive, Catalina^ higher split the drive into Macintosh HD & Macintosh HD Data, with a reserved 1.3 MB Update Partition used durn Updates.
BDAqua - have used this for Sierra years and years ago.
But, have seen some having issue with Path in the command.
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app --nointeraction &&say Done
Well, made it through. I just kept trying things until it worked. Had to eventually install it (Sierra) to a device/drive called "UPDATE" and that was the only one that would work. Of course, now I have three devices in Finder called "Macintosh HD -- Data" for whatever reason. . . . .
Thank you all!
Abject failure so far. I can't get the bootable installer created -- none of the command line entries I put in have worked -- and any other method I try only lets me reinstall Big Sur.
Can you show us the exact terminal command you used?
Aha, the last try got it. . . . . .I made a clean install of Big Sur on it. It is currently Copying installer files to disk. . . . .
Am at the "The operation couldn't be completed. (com.apple.osinstall error -3.)"
The file system?
Backwards restore from 11.6.5 to 10.12.6
