Boot disk

A boot disk is a removable digital data storage medium from which a computer can load and run (boot) an operating system or utility program. The computer must have a built-in program which will load and execute a program from a boot disk meeting certain standards.

While almost all modern computers can boot from a hard drive containing the operating system and other software, they are not normally called boot disks because they are not removable media. Bootable fixed drives are called boot drives. CD-ROMs were the most common boot disk media, but have mostly been replaced by USB flash drives. Other media, such as magnetic or paper tape drives, or ZIP drives may be supported by older devices. The computer's BIOS must support booting from the device in question.

Boot disks can be used to recover a device if it won't start properly.