Your 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 an external disk or a CD or DVD that has system software on it. See Change your Mac startup disk.
To find out which disk is being used as the startup disk, choose Apple menu 
> System Preferences, then click Startup Disk.
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, then choose File > Get Info. To increase available space, you can compress folders, delete personal items, or store items on a separate disk. To learn more about freeing up storage, see Optimize storage space in System Information.
Partition a disk
You can divide a single disk into sections, called partitions or volumes, that act like smaller disks. Some people do this to help organize and manage their disk space. You can create partitions using Disk Utility.
Back up your disk
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 disk can be replaced.