Clean install macOS without a USB Drive Since Catalina it has been possible to reinstall macOS without having to boot to a USB drive or creating a new partition with the installer. It is just a matter of some simple Terminal commands. Be sure to make a Time Machine backup first because everything will be deleted!!! First we download the installer app, then we start a new installation. Open Terminal and type the following command: Download latest installer: sudo softwareupdate --fetch-full-installer When the installer is downloaded you proceed with the following command, depending on which installer was downloaded: Install Big Sur: /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/startosinstall --eraseinstall --agreetolicense --forcequitapps --newvolumename 'Macintosh HD' Install Catalina: /Applications/Install macOS Catalina.app/Contents/Resources/startosinstall --eraseinstall --agreetolicense --forcequitapps --newvolumename 'Macintosh HD'