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I'm following instructions on how to downgrade from MacOS catalina back to Mojave as Catalina doesn't support ProTools. I was following the instructions fine and then I got stuck in the process of making an external bootable hard drive. The webpage I'm reading instructs to make the drive in APFS format if my laptop is in that format and in MacOS extended journaled if my Mac is in HFS+ format. Well my Mac is in APFS format so that's the format I chose for the external hard drive. However during this next step in terminal, I'm getting an error message. APFS disks may not be used as bootable install media. An error occurred erasing the disk. But it would work if the hard drive was in MacOS extended journal format. My question is will my laptop launch MOJAVE from a hard drive in HFS+ format when my laptop is in APFS format? Or will that screw everything up?

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  • Hello Amanda! Welcome to the site. What tool are you using to create the external bootable hard drive? Also, can you link to the instructions you are using? It can be helpful to make sure we're looking at the same thing you are.
    – TJ Luoma
    Commented Nov 12, 2019 at 5:47

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The Apple documentation How to create a bootable installer for macOS says to use Mac OS Extended not APFS.

Mac OS Extended is the new name for HFS+

Use the 'createinstallmedia' command in Terminal

  1. Connect the USB flash drive or other volume you're using for the bootable installer. Make sure that it has at least 12GB of available storage and is formatted as Mac OS Extended.
  2. Open Terminal, which is in the Utilities folder of your Applications folder.

  3. Type or paste one of the following commands in Terminal. These assume that the installer is still in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If it has a different name, replace MyVolume in these commands with the name of your volume.

    Catalina:*

    sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

    Mojave:*

    sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

From the link in the above quote

Are you preparing a Time Machine backup disk or bootable installer?

Choose Mac OS Extended (Journaled) for any disk that you plan to use with Time Machine or as a bootable installer.

As you can see the documentation is somewhat inconsistent regarding journaling (it probably doesn't matter) however I used Mac OS Extended (Journaled) to install Mojave and it worked fine.

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  • when I run the createinstallmedia I get the error 'APFS disks may not be used as bootable install media.' the target volume is a partition I created with DiskUtil, and the newly created partition info shows 'Bootable:No'
    – izzy
    Commented Apr 14, 2020 at 1:26
  • @izzy - you must format your installer volume macOS extended not APFS.
    – lx07
    Commented Apr 14, 2020 at 5:34
  • I just did the factory reset, it was command shift R or something when going into recovery mode
    – izzy
    Commented Jun 1, 2020 at 10:39

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