My MacBook’s battery capacity is significantly reduced and it has many weird issues so I decide to do a clean installation of macOS Big Sur. This article is a reference for my own reference in case I need to do it again.


I plan to erase the disk and install a clean copy of macOS, so the first step is creating the bootable installer.

  • Back up the data. Follow these instructions if I want to sell, give away, or trade in my Mac.

  • Get Big Sur from the App Store. This will install the app Install macOS Big into /Applications.

  • Insert the USB stick which has at least 8GB capacity. Assuming the stick name is MyVolume, create the installer using the USB stick with following command 1

    sudo /Applications/Install\ macOS\ Big\ --downloadassets --volume /Volumes/MyVolume

    The option --downloadassets is optional but is recommended, because without proxy, it is slow to download resources during the installating stage in some regions.

  • Reboot the laptop into recovery mode by holding Command+R. Choose Utilities > Startup Security Utility from the menu bar and allow booting from external media.2 This step is very important and must be done before erasing the disk because it requires verifying the admin user password in the installed system. I fogot it and had to recover the factory macOS version to just create a user so that I can allow booting from external media.

  • Ensure there’s a working Internet connection. The installation requires network to do verification.


Keep the USB stick inserted. Reboot the laptop by holding Option (⌥) this time. This will bring the startup disk selection interface. Select the USB stick, which is shown as “Install Big Sur”.

This will boot into the Big Sur recovery mode.

  • Use disk utility to erase the disk. Use security options to securely erase the data from the disk if I plan to sell the Mac or send it to the genius bar.
  • Quit disk utility and choose to install Big Sur from the menu.