


If there are multiple icons showing up on your Mac, it means that you have multiple disks on your Mac. To figure it out on your Mac, you can first make all disks show up on your desktop: click Finder on the Dock, select Preferences, and check "Hard disks". Usually, there is only one startup disk on a Mac, but it is also possible that you have divided your hard drive into different disks and get multiple startup disks. By default, it’s named "Macintosh HD", although you can freely change it.

Simply put, a startup disk on Mac is a disk with your operating system(such as macOS Mojave) on it.
