Create a Bootable USB Stick from an Older Version of macOS

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Download the version of the OS you want to install here: https://support.apple.com/en-gb/HT211683

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Open InstallMacOSX.dmg by double-clicking.

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If you don't get the following error: "This version of OS X/macOS cannot be installed on this computer."

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Quit the installer and drag and drop the InstallMacOSX.pkg file on your Desktop

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Open Terminal and enter the following command: "cd Desktop"

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Open the folder appeared on the desktop.

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Go back to the terminal and type the following command: pkgutil --flatten FolderName New.pkg (FolderName = the name of the folder and New.pkg = the new .PKG file that will be created)