Alternatively referred to as boot options or boot order, the boot sequence defines which devices a computer should check for the operating system's boot files. It also specifies in which order those devices should be checked. The list can be changed and re-ordered in the computer's BIOS, as shown in the example below.
In the above example, the computer first looks at removable devices (e.g. floppy diskette drive) to see if it can be booted. If nothing was in the drive or it wasn't bootable, the computer next looks at the hard drive. If an operating system such as Windows is installed, it boots into Windows.
In the above example, if you were trying to reinstall Windows, this boot sequence would not work because the computer would try booting from the hard drive before the CD-ROM. To boot from the Windows CD and setup Windows, the CD-ROM or disc drive must be before the hard drive.
The most common devices to be listed in the boot sequence are the floppy (3.5") drive, disc drive (CD or DVD) drive, and hard drive. More recent computers may also contain a USB storage device in the boot sequence. When installing an operating system, the boot sequence would need to include a CD or DVD drive, or possibly a USB storage device with a higher priority than a hard drive. After the operating system is installed, the hard drive could be a higher priority in the boot sequence.