An optical disc (CD, DVD) or USB drive that contains a bootable program that takes control of the computer. Computers are typically configured to look for the OS on a CD or DVD first and then the hard disk or SSD. The computer can also be configured to look for the USB drive first, although very early computers do not have that option.
System and Data Recovery
A bootable disk is used to recover a failed system when the OS on the internal storage drive does not load. The OS on the bootable disk can be a very lightweight version of the OS running in the computer, or it may be an entirely different OS. In the past, a compact version of DOS was used.
The bootable OS must support the file system on the computer's internal drive, and it must have sufficient drivers for the peripherals used in the recovery. In addition, bootable recovery disks contain troubleshooting and repair utilities. Even if the operating system cannot be restored, the bootable disk often enables valuable data files to be copied to an external drive. See file system
Install or Replace an OS
A bootable disk is also used to install a new operating system. In this case, it boots a small program that is able to overwrite the existing OS or install the OS from scratch. For brand new computers, it often has an option to format the storage drive. The installation program on the bootable disk has complete control of the computer. See LiveCD
and ISO image