Install macOS 10.15 using a bootable installer

    1. Get an external hard drive that has at least 12 GB of available storage and plug it in your Mac.
    2. Launch the Disk Utility app and erase your hard drive selecting Mac OS Extended format.
    3. Open Terminal (Applications > Utilities).
    4. Paste this command and press Enter:
  1. sudo /Applications/Install\ macOS\ --volume /Volumes/MyVolume
  2. If prompted, enter your administrator password and press Enter again.
  3. When Terminal says “done”, your hard drive should have the same name as the installer you downloaded (for example, Install macOS Catalina).
  4. Open System Preferences > Startup Disk. Choose your bootable installer as a startup disk and press Restart.
  5. Your Mac should start up to macOS Recovery.
  6. Make sure your Mac has an internet connection to download firmware updates (you can use the Wi-Fi menu in the menu bar).
  7. In the Utilities window, select Install macOS.
  8. Your Mac will then start installing macOS Catalina and will restart when it’s done.