In order for a hard disk or disk partition to be accessible by a computer, it must first be mounted. This is a software process that "activates" the disk, which makes the folders and files on the disk readable by the computer. If a hard drive is physically connected, but not mounted, the computer will not recognize it.
Fortunately, most operating systems, such as Windows and Mac OS X, mount newly connected disks by default. In Windows, the disk will appear in the "My Computer" window. In Mac OS X, the disk will appear on the desktop. This is true for all types of disks, such as internal and external hard drives, optical media, such as CDs and DVDs, and USB flash drives. Disk image files may have to be mounted manually using a program like Nero or WinImage (Windows) or Apple Disk Utility (Mac OS X).
If a disk is not being used, it can be unmounted. Optical disks are typically unmounted automatically when they are ejected. However, before you disconnect an external hard drive or a USB flash drive, you should unmount the disk to avoid possible data corruption.