A CD-ROM or DVD-ROM that contains its own operating system ready to load. PCs are typically configured to look for the OS on a CD or DVD first and then the hard disk. Newer machines can also boot from a flash memory-based USB drive (see USB drive). System and Data Recovery A bootable disk is used to recover a failed system when the OS on the hard disk cannot load. The OS on the bootable disk does not have to be the same as the OS on the hard disk. However, it must support the file system on the hard disk, so that hard disk files can be examined and repaired. In addition, the OS on the bootable disk must contain the appropriate software drivers for all the peripherals that will be used in a recovery attempt. If the system cannot be restored, valuable data files can be copied to an external drive. In the past, a compact version of DOS was often used as a bootable disk. Install or Replace an Operating System A bootable disk is also used to install a new operating system. By booting its own operating system (not necessarily the one being installed), it is able to format and write to the blank hard disk on a new computer. In addition, when replacing an existing operating system, it gives the installation program complete control of the computer. See LiveCD.