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I want to download Big Sur from an older Mac to re-install on my 2017 iMac

I need to re-install Big Sur on my 27 iMac. It will not restart, only in safe mode. I tried reinstalling from a Timemachine backup but it will not restart. I can not install a MacOS form the safe mode as it shows only High Sierra (the original OS on the machine). Since I have theoritically Big Sur from my Time Machine backup, the system does not want to re-install even High Sierra as a more recent OS is on the drive (Big Sur).


So i want to use my old 2011 Macbook Pro to create a bootable drive to do a fresh install on my iMac... That Mac being on High Sierra, it does not want to download Big Sur as it is not compatible... I just need to download an installer of some sort!


Sounds a bit confusing, I know.

iMac 27″ 5K, macOS 11.6

Posted on Nov 24, 2021 5:59 PM

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Posted on Nov 25, 2021 6:43 AM

Sorry to read having so many issues.


From the series of events, in you reply, it would appear the Big Sur has not installed but is occupying a lot of space. Thus, the restarting and shutting down before it fully loads.


There might be a way to clear off some files by using Target Disk Mode


That, or on the older 2011 computer - make a a bootable installer of High Sierra ( yes going backwards ) and use that to Wipe the iMac back to High Sierra and commence thereafter the upgrade to Big Sur.


The below method will Wipe ALL DATA on the iMac including the Recovery Volume.


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 High Sierra would have to be done on a computer that Qualifies to run High Sierra.


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 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.



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Question marked as Best reply

Nov 25, 2021 6:43 AM in response to PVSherbrooke

Sorry to read having so many issues.


From the series of events, in you reply, it would appear the Big Sur has not installed but is occupying a lot of space. Thus, the restarting and shutting down before it fully loads.


There might be a way to clear off some files by using Target Disk Mode


That, or on the older 2011 computer - make a a bootable installer of High Sierra ( yes going backwards ) and use that to Wipe the iMac back to High Sierra and commence thereafter the upgrade to Big Sur.


The below method will Wipe ALL DATA on the iMac including the Recovery Volume.


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 High Sierra would have to be done on a computer that Qualifies to run High Sierra.


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 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.



Nov 25, 2021 2:33 AM in response to PVSherbrooke

If the computer will start in Safe Mode, it appear the computer is still functioning - good sign. When in this mode - it it booting into Big Sur or High Sierra ? If it boots into Big Sur >>>


Restart the computer in Recovery Mode using the Command + r keys immediately at restart. This will allow one to choose Reinstall macOS and basically download and install Big Sur over the top of the existing Big Sur installation. This will replace and corrupted or missing elements of Big Sur


Notation - the older computer from 2011 does not Qualify to run Big Sur and the Apple Store will not even allow one to download the Full Version of Big Sur.

Nov 25, 2021 6:03 AM in response to PRP_53

Thanks for the reply. The iMac actually only restarts in Recovery mode, not safe mode, my mistake. I did try reinstalling the MacOS but it offers only High Sierra. And it shows me a message that it can’t install High Sierra because the disk has a more recent OS installed.


this is why I want to get a fresh copy of Big Sur on a bootable drive somehow.


as a note, if I restart the computer, it does start, shows the progress bar half full, and then shuts down

I want to download Big Sur from an older Mac to re-install on my 2017 iMac

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