How to Downgrade from MacOS Catalina to Mojave- Creating a bootable hard drive in APFS format 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?
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
  
  
  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.
  Open Terminal, which is in the Utilities folder of your Applications folder.
  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.