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Terminology: Volume vs Drive
A drive (or disk) is the physical device such as hard drive or a PCIe card with storage on it, a flash drive (SSD) or an external enclosure which presents itself as a single drive, or passes through all drives in the box, or some combination thereof.
A portion of the space on a drive is called a partition. When initialized to contain a file system, the result is a volume, which is what shows up on your desktop.
A Mac from Apple is delivered with one user-visible volume on one drive, typically “Macintosh HD” (“HD” is misleading, since many Macs now come with flash drives).
A volume is logical storage space that contains the file system. The file system is the catalog (index) that locates all the files and folders found on the volume. The file system structure is oblivious to the hardware structure underneath; it could be one drive or fifty drives and portions thereof.
Apple confuses this issue by making a mess of it: your startup “disk” really means startup volume, but infor the volume it says “ ”.
Of course, it could be a hard drive or a flash drive/SSD, or a network volume or something else! Misused terminology leads to unnecessary confusion. And sometimes lost data with the frustrating trend to hiding things in obscure places in obscure formats (e.g. OS X Calendar data).
Volumes show up as icons
The volume icon Master as shown at right could be just about anything:
- One partition taking up all (nearly all) the space on a drive.
- A partition of drive taking up some small fraction of the drive.
- A RAID-0 stripe, a RAID-1 mirror, a RAID-5 or RAID 1+0.
- A volume composed of 16-drives in some external enclosure.
- A USB thumb drive
What a user works with and sees on the desktop is a volume; never a drive or disk.
Example with ten volumes
See How to Partition Drives Into Multiple Volumes for reasons to use partitions.
Hardly anyone needs ten volumes, but a software developer looking to boot from ten different OS X versions (for testing) might well want to have ten volumes, or more.
As shown below, one (1) drive has been partitioned into ten (10) volumes.
The 10 volume icons above are all on the same single physical drive.
Mouse over the start/confirm/status buttons to see the process.