Learn about your Mac computer’s internal disk
Your Mac has an internal disk, which is a storage device that contains the apps and information used by your Mac. Some Mac computers have additional internal disks or connected external disks.
Specify a startup disk
The startup disk contains the macOS system software. Your internal disk is usually the startup disk, but you can also start up your Mac from a network disk or an external disk that has system software on it. See Change your Mac startup disk.
Important: If you have a Mac with an Apple T2 Security Chip, added security features may be set to prevent the computer from starting up from an external startup disk. See What is the Startup Security Utility? If you have a Mac with Apple silicon, see Change the security policy.
To find out which disk is being used as the startup disk, choose Apple menu 
in the sidebar, then click Startup Disk on the right (you may need to scroll down).
Manage disk space
To see how much space is available on a disk, open a Finder window, select the disk in the Locations section of the Finder sidebar, choose File > Get Info, then click 
next to General. To increase available space, you can compress folders, delete personal items, or store items on a separate disk. See Optimize storage space.
Divide your internal disk into different sections
You can divide a single disk into sections, called partitions or volumes, or containers that act like smaller disks. Some people do this to help organize and manage their disk space. You can create or remove APFS volumes using Disk Utility.
Back up your disk information
It is very important to regularly back up the files on your internal disk. Time Machine makes backing up easy. See Back up your files with Time Machine.
Troubleshoot disk problems
If you suspect or experience problems with your internal disk, Disk Utility can help you diagnose and repair problems. Be sure to follow instructions carefully.
If necessary, a faulty hard disk drive (HDD) can be replaced. For service information, see Find out how to service or repair your Mac.