Maybe I can help out.
Lets think of a freeway for a moment. All of the cars are depending on the road surface to be a smooth as possible so that their driving experience will not cause damage to their vehicle. On the road, there are signs to assist on direction: knowing when the next off-ramp is and where it will take you. There are also lines in the road to provide lanes for the cars to travel. Over time, the roads and signs need repair. Potholes, graffiti and maybe even a little construction to improve the way it looks. Before the repairs are made, the city has to take some time to assure that it is done properly: permits, contractors, ect. In the meantime, vehicles are still using the roads. Damage may be minimal, but results and be slow to realize. The potholes become bigger, causing damage to vehicles overtime (the suspension and such), or the graffiti may distract drivers and cause accidents. The results are the same: vehicles (in cases of accidents, people injured) are not being used to their potential. By the time that the repairs are finished, another portion of the freeway may need some repair.
Using these terms, the freeway is the physical surface of your hard drive-the platters. The vehicles is the data being moved around (ironically at nearly car-like speeds). The repair process would be the scan disk that is being ran. Even though the data on the disk is stationary during the scan disk process (think:traffic jam), some of the data is needed to run the scandisk application. When it finds a portion of the hd that is corrupt, it has to find another place to move the data and physically put it there. I dont know about you, but if I am in a traffic jam, I wouldn't mind being air lifted out of it
We are all curious like a cat. We wonder, we ask, we learn.
Please post back when a suggestion works, so that others may learn.