Choosing a startup disk on macOS, also known as the system disk or boot disk, is the process of selecting the storage device that the computer will use to load the operating system and start up. The startup disk is typically an internal hard drive or solid-state drive (SSD), but it can also be an external drive or a network volume.
It is important to choose the correct startup disk, as the computer will not be able to start up if the selected disk does not contain a bootable operating system. Additionally, choosing the fastest startup disk will improve the overall performance of the computer.