A boot disk is a removable digital data storage medium, normally read-only, that can load (boot) an operating system or utility program.
Boot disks may create and use a RAM disk for temporary file storage, to eliminate the need to write to the fixed storage media of the computer where the boot disk is used.
Boot disks are used for:
Operating system installation.
Data recovery.
Hardware or software troubleshooting.
Customizing an operating environment.
Software demonstration.
Floppy disks and CD-ROMs are the most common forms of media used, but other media, such as tape drives, zip drives and more recently, USB flash drives can be used. What can be booted depends on whether a computer's BIOS supports booting from that device. For example, some BIOSes do not support booting from a CD-ROM and require a boot floppy to run the software from the CD-ROM.
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Booting from a disk
A modern PC is configured to attempt to boot from various devices in a certain order. If your computer is not booting from the device you desire, such as the floppy drive, you must enter the BIOS setup function, by pressing a special key when the computer is first turned on -- Delete, F1, F2, or F10. There will be a menu option for changing the boot order.
http://www.bootdisk.com/bootdisk.htm