Volumes

The idea that root is at the top of the file structure is true for a single disk, but what happens if you add another disk to your system? Each disk has its own directory structure, so each disk has its own root.

When describing file systems, disks are called volumes. This is because not all storage devices (that is, volumes of data) are disks: they might be USB sticks, DVD drives, storage on the network, or even (common in the 1970s and 80s) tape drives.

• Adding a new volume to a file system is called mounting it.

• Removing a volume is unmounting it.

Volumes on Unix

On Unix systems, mounted volumes appear as directories within the root of your main system. The place when the files appear is called the mount point. The directory structure of that volume appears — just like the other directory trees — as subdirectories of that mount point. The mount point is effectively the root of that volume.

Mount points are conventionally stored in either the /mnt or /media directories of the system they’ve been mounted on.

The Unix mount and unmount commands are used to do this — in normal operation you probably won’t need to do this because it’s done by the system administrators. If you’re running Unix on your own laptop (or a Mac) then the chances are the operating system will automatically do this for you when it detects you’ve plugged in a new storage device (such as a USB stick).

Volumes on Windows

Windows does things differently, and doesn’t mount volumes in this way. Instead it uses drive letters, followed by a colon (:). The main drive is conventionally called C:.

Other volumes each have a different drive letter.

This means that you need to explicitly specify drive letters when referencing other volumes on Windows; they are present as discrete file systems. On Unix everything is presented as a single file system.