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Create an OSx rescue USB using a Modern M1 Mac

We are going to temporarily create an image on the desktop. Make sure you have sufficient disk space. This temporary  directory will be deleted and a MacOSx rescue disk will have been created in the end of the tutorial.

Successfully tested on a Mac M1: Mac OS High-Sierra, El Capitan.

Start by downloading the OS-X dmg from this link :

Then open the dmg and copy InstallMacOSX.pkg on (in this example) the Desktop.

Then, from the terminal (Applications/Utilities) un-compact the InstallMacOSX.pkg file in a directory (Installer for example) which will be created by the following pkgutil command:

Note: please always check the commands before u put something in your terminal!

pkgutil --expand ~/Desktop/InstallMacOSX.pkg ~/Desktop/Installer

Then position yourself in the InstallMacOSX.pkg “package” created by the pkgutil command

cd ~/Desktop/Installer/InstallMacOSX.pkg

Then un-compact the structure using the tar command:

tar -xvf Payload

Finally move the InstallESD.dmg file created by the tar command above to the Desktop

mv InstallESD.dmg ~/Desktop

You must then format a GUID partition scheme USB key of sufficient size named KEY (or any name, just make sure to edit the last command) in the example and execute the following instructions :

hdiutil attach ~/Desktop/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_app 
hdiutil convert /Volumes/install_app/BaseSystem.dmg -format UDSP -o /tmp/Installer 
hdiutil resize -size 8g /tmp/Installer.sparseimage 
hdiutil attach /tmp/Installer.sparseimage -noverify -nobrowse -mountpoint /Volumes/install_build 

rm -r /Volumes/install_build/System/Installation/Packages 
cp -av /Volumes/install_app/Packages /Volumes/install_build/System/Installation/ 
cp -av /Volumes/install_app/BaseSystem.chunklist /Volumes/install_build 
cp -av /Volumes/install_app/BaseSystem.dmg /Volumes/install_build 

hdiutil detach /Volumes/install_app 
hdiutil detach /Volumes/install_build 
hdiutil resize -size `hdiutil resize -limits /tmp/Installer.sparseimage | tail -n 1 | awk '{print $ 1}' `b /tmp/Installer.sparseimage 
hdiutil convert /tmp/Installer.sparseimage -format UDZO -o /tmp/Installer 

mv /tmp/Installer.dmg ~/Desktop

Here you have to plug the USB named KEY (or any name and rename “KEY” in the command below), then :

sudo asr restore --source ~/Desktop/Installer.dmg --target /Volumes/KEY --noprompt --noverify --erase

Test the key and if ok, delete the working directories and files of this operation from the Desktop.