What is a functional disk?

A disk is functional if it is recognized by your computer's BIOS as "Drive n" in setup, or by your operating system, usually as drive "C:", "D:", ...  Even though your computer's operating system may not be able to start (boot) from a disk, or show that a disk is present, it may still be functional. 

When powered on, your computer's BIOS will attempt to power-on all your disk drives.  Once the drives "spin-up", the disk controller will verify that the disk is in a "ready" condition, and will notify the BIOS.  The condition of the drives will then be shown in BIOS setup.  If BIOS setup does not list the disk in question, it is likely that components of the disk subsystem (disk, cable, +5/+12VDC power, or controller) have failed; usually, the disk drive is the failing component.