A USB stick with at least 8 GB of memory (16 GB recommended). An ISO or DMG installation image of a version of Mac OS (you can find them at this link). This guide is valid only for versions after 10.5. Download the latest version of TransMac from this link. Once downloaded connect your USB drive. Dec 25, 2019 Make sure the system you are re-installing Catalina on is powered off and insert the USB stick called “Install macOS Catalina” into the system. Power the system on and push and hold the “option” key while it is powering on. If your Mac still refuses to boot off your USB stick you may find it easier to boot and install off an Ubuntu DVD instead. See our How to burn a DVD on macOS for further details. Alternatively, if you feel confident using the macOS command line, see the community documentation on How to install Ubuntu on MacBook using USB Stick for a more. Apple doesn't sell Lion on a disk or flash drive, which doesn't give you much of an option to perform a clean install. In this video, I show you exactly how.