If it fails, try again-sometimes the system doesn't unmount the volume in time and the process will be unable to complete.
Turn on your Mac and continue to hold the power button until you see the startup options window, which shows your bootable volumes. Some Linux variants may require larger volumes, so pay attention to the requirements when downloading. Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing. Replace /path/to/downloaded.iso with the location of your downloaded Linux ISO, for example if ubuntu.iso is in your Downloads folder, you can type /Downloads/ubuntu.iso. When you're looking to create a bootable Linux USB drive on a Mac, the first step is to make sure you've got the right USB drive for the job, and that it's formatted correctly to avoid any problems. Open Terminal and enter the following command: hdiutil convert /path/to/downloaded.iso -format UDRW -o /path/to/image.