Manage volumes and partitions
The word volume is used to describe a storage device, like a hard disk. It can also refer to a part of the storage on that device, because you can split the storage up into chunks. The computer makes this storage accessible via your file system in a process referred to as mounting. Mounted volumes may be hard drives, USB drives, DVD-RWs, SD cards, and other media. If a volume is currently mounted, you can read (and possibly write) files on it.
Often, a mounted volume is called a partition, though they are not necessarily the same thing. A “partition” refers to a physical area of storage on a single disk drive. Once a partition has been mounted, it can be referred to as a volume because you can access the files on it. You can think of volumes as the labeled, accessible “storefronts” to the functional “back rooms” of partitions and drives.
View and manage volumes and partitions using the disk utility
You can check and modify your computer's storage volumes with the disk utility.
Open the dash and start the Disk Utility.
In the pane marked Storage Devices, you will find hard disks, CD/DVD drives, and other physical devices. Click the device you want to inspect.
In the right pane, the area labeled Volumes provides a visual breakdown of the volumes and partitions present on the selected device. It also contains a variety of tools used to manage these volumes.
Be careful: it is possible to completely erase the data on your disk with these utilities.
Your computer most likely has at least one primary partition and a single swap partition. The swap partition is used by the operating system for memory management, and is rarely mounted. The primary partition contains your operating system, applications, settings, and personal files. These files can also be distributed among multiple partitions for security or convenience.
One primary partition must contain information that your computer uses to start up, or boot. For this reason it is sometimes called a boot partition, or boot volume. To determine if a volume is bootable, look at its Partition Flags in the disk utility. External media such as USB drives and CDs may also contain a bootable volume.