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I have been fighting to downgrade my MacBook Pro 2012 to mountain lion. I accidentally let it update to catilina. I struggled to create the bootable usb according to all the videos online (ended up using super duper to create one) now when i boot with option I get into the mountain lion installer I can’t see my hard drive I only have the option to install to the the usb which won’t work either

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A quick check at everymac.com shows the earliest OS X that can be installed on a 2012 MacBook Pro model is between OS X 10.7 3 and OS X 10.8.2 inclusively. Therefore, you should be able to install Mountain Lion on your Mac. However, everymac.com shows the earliest OS X that can installed on my iMac (21.5-inch, Late 2013) is OS X 10.8.4 and I can not boot from any OS X 10.8.5 installer created using instructions given in this answer. My 2011 iMac will boot without any issues.

Catalina uses APFS which will not be recognized by Mountain Lion. After booting from the Mountain Lion installer, you will need to open the Disk Utility and erase the entire internal drive. You should choose "Mac OS Extended (Journaled)" as the format. Use "GUID Partition Map" as the scheme.

Since you have stated you have correctly created the USB Mountain Lion installer, I will not post how to do so. However, if you are still unable to install, perhaps you should include in your question the steps you used to create the installer.


Update 1: Test Install of Mountain Lion

I did download the InstallMacOSX.dmg file for OS X 10.8.5 (Mountain Lion) from How to download macOS on a iMac (21.5-inch, Late 2013) with macOS 10.15.7 (Catalina) installed. I was able to create a Mountain Lion bootable installer from this downloaded file. I did not need to use any third party tools (such as SuperDuper) to create the installer. I used the bootable installer to install Mount Lion in a VMware Fusion Player (Version 12.1.0) virtual machine. After installing, I was able to download and install all available updates except for an update to iTunes 12.14.3. I was able to download this update on the host and transfer the update to the virtual machine where installation was successful.


Update 2: How to Get the InstallESD.dmg File from Apple

This was tested using macOS Catalina (10.15.7).

  1. From How to download macOS, download the InstallMacOSX.dmg file for OS X 10.8.5 (Mountain Lion) to your ~/Downloads folder.

  2. Enter the command below to open InstallMacOSX.dmg. This will mount the readonly Mac OS Extended (HFS+) volume on /Volumes/Install Mac OS X.

    hdiutil attach ~/Downloads/InstallMacOSX.dmg 
    
  3. Enter the command below to extract folders and files from InstallMacOSX.dmg to the ~/Downloads/pkg folder. Here, I assume the ~/Downloads/pkg folder does not already exist.

    pkgutil --expand /Volumes/Install\ Mac\ OS\ X/InstallMacOSX.pkg ~/Downloads/pkg
    
  4. Enter the command below to eject /Volumes/Install Mac OS X.

     diskutil eject /Volumes/Install\ Mac\ OS\ X/
    
  5. Use the command below to move the InstallESD.dmg file to the ~/Downloads folder.

    mv ~/Downloads/pkg/InstallMacOSX.pkg/InstallESD.dmg ~/Downloads/
    
  6. Use the Finder application to move the ~/Downloads/pkg folder to the Trash.

  7. (Optional). To save space, you can move the downloaded file ~/Downloads/InstallMacOSX.dmg to the Trash, then empty the Trash.

I should point out that the InstallMacOSX.dmg file can be used to create an Install OS X Mountain Lion application. This application also contains an InstallESD.dmg file in the Contents/SharedSupport folder. While these two InstallESD.dmg files are different sizes, the contents are identical.


Update 3: Creating a APM USB Flash Drive Installer

I was unable to use the Disk Utility included with Catalina to create a bootable USB flash drive installer. So I used the following steps to transfer an exact image from the InstallESD.dmg file to the flash drive.

  1. Insert a 8 GB or larger USB flash drive.

  2. Enter the command below to open InstallESD.dmg file. This will mount the readonly Mac OS Extended (HFS+) volume on /Volumes/Mac OS X Install ESD.

    hdiutil attach ~/Downloads/InstallESD.dmg
    
  3. Enter the command below to determined the identifiers for the external, physical USB flash drive and the disk image. In my case, the flash drive has been assigned disk5 and the disk image has been assigned disk6. If your assignments are different, then make the appropriate substitutions in the following steps.

    diskutil list
    
  4. Unmount all volumes on both the flash drive and disk image by entering the commands given below.

    diskutil unmountdisk disk5
    diskutil unmountdisk disk6
    
  5. Enter the command below to transfer a exact copy of the disk image to the flash drive.

    Note: The transfer occurs faster when the identifiers are proceeded by the letter r.

    sudo dd if=/dev/rdisk6 of=/dev/rdisk5 bs=1m
    
  6. Enter the command below to eject the mounted disk image.

    diskutil eject disk6
    

Note: The flash drive is using the Apple Partition Map (APM).

Update 4: Converting from APM to GPT

If your Mac can not boot from a flash drive using the Apple Partition Map (APM), then use the following steps to covert to a GUID Partition Table (GPT).

  1. Enter the command below to get the beginning partition offset in sectors (512-Byte-Device-Blocks) of the volume labeled Mac OS X Install ESD on the USB flash drive installer. Also, get the disk (partition) size in sectors (512-Byte-Units). If your identifier for the USB flash drive installer differs from disk5, then make the appropriate substitution.

    diskutil info disk5s2 | grep -e Offset -e Size
    
       Partition Offset:          32768 Bytes (64 512-Byte-Device-Blocks)
       Disk Size:                 4.9 GB (4856860672 Bytes) (exactly 9486056 512-Byte-Units)
       Device Block Size:         512 Bytes
       Allocation Block Size:     4096 Bytes
    

     

    Note: You can get the same values by using the sudo pdisk -l /dev/disk5 command. However, you will need to multiply the base and length values by 4 to convert to the Partition Offset and Disk Size 512 byte sector values shown above.

  2. Enter the commands below to create the GUID Partition Table (GPT). Use the offset and size values determined in the previous step. If your values are different for the identifier, offset and/or size, then make the appropriate substitutions. The gpt destroy command may generate an error message which can be ignored.

    Note: The offset follows the -b option and the size follows the -s option.

    diskutil unmountdisk disk5
    sudo gpt destroy disk5
    diskutil unmountdisk disk5
    sudo gpt create disk5
    sudo gpt add -i 1 -b 64 -s 9486056 -t hfs disk5 
    

    The rest of the steps are optional.

  3. Turn on journalling.

    diskutil enableJournal disk5s1
    
  4. Add free space to the volume.

    diskutil resizevolume disk5s1 R
    
  5. Rename the volume.

    diskutil renamevolume disk5s1 "Install Mountain Lion"
    
  6. Add an icon which appears in the Mac Startup Manager on some Macs.

    cp /Volumes/Install\ Mountain\ Lion/Install\ OS\ X\ Mountain\ Lion.app/Contents/Resources/InstallAssistant.icns /Volumes/Install\ Mountain\ Lion/.VolumeIcon.icns
    
  7. Change the label. The current label which appears in the Mac Startup Manager is Mac OS X. The command below changes this label to Install Mountain Lion.

    sudo bless --folder /Volumes/Install\ Mountain\ Lion --label Install\ Mountain\ Lion 
    
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  • To create the usb I followed these instructions except when it got to restoring in disk utility as it continuously failed. then used super Duper’s restore function to create the USB with that ESD installer. I can boot the mountain lion installer, but the usb is still labelled untitled
    – Kane Smith
    Commented Jan 5, 2023 at 22:32
  • @KaneSmith: What are "these instructions"? Did you intend to include a link in your previous comment? Commented Jan 5, 2023 at 22:41
  • lifewire.com/…
    – Kane Smith
    Commented Jan 6, 2023 at 16:41

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