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Once you start the computer it will search for the boot device to load the operating system. On most systems this is done from hard drive. The moment POST is completed the computer searches for the boot disk or bootable hard drive. If you have OS installed in the computer then it will boot from hard disk else it will throw an error- No bootable device found.
The boot disk contains certain files that is necessary or that helps the computer load the operating system. These files include Command.com, MSDOS.SYS, IO.SYS etc. A boot disk is a handy tool if you have problem starting a computer. It can help you boot the computer even without the hard disk. The bootable disk can be a CD or a floppy disk. It will also help you diagnose startup or booting problems.
