
Every colored section represents a category.
Click the Storage tab, and you will see that the storage bar is divided into sections. Click on the Apple icon in the upper left corner of your screen and choose About This Mac. To check the usage of your boot disk, follow the steps below: When the system says that your startup disk is almost full, you need to pay attention to it. If you are one of the few users who have 2 or more disks in Mac, you are free to choose which one to be your startup disk.
Thus, the Macintosh HD is the default startup disk. You can check your startup disk by clicking the Apple menu > System Preferences > Startup Disk.
The disk where your current macOS is installed is the startup disk. How to Delete System Data on Mac What is Startup Disk on Mac?