Hard drive media damage
Media damage is caused when the read/write heads punch through the air bearing (created when the drive spins up)
and contacts the surface of the platters. When this happens, the heads scratch
the surface destroying needed information for the drive to work at all, not to mention destroying your data.
This photo represents severe surface damage. In this particular case, the drive did not shut down after the initial failure.
The drive continued to spin and Head Rack assembly continued to traverse the media causing the mottled look that you see.
Your drive may be no where near this damaged but any visible concentric scoring of the media renders the drive unrecoverable.