If you want to re-install macOS on a running older Mac or have no usb or dvd boot option: Shrink boot drive by size of the macOS Install DMG. 5 or 6 gb Create new partition with new free space Restore macOS install DMG to new partition space. Go to preferences > startup and select your "macOS install" as startup. Reboot. Install macOS to bootable partition. Works on new macs, older macs and even Windozzz Tested on my mac mini G4 which has no usb boot option and Leopard over 4.7 gb dvd.