What exactly is head crash or hard disk crash?
The inner workings of a hard disk are such that the read-write head FLOATS above the platters (where your data is stored) during operation at the amazing height of 0.5 microinches! Your hair in comparison is over 2,000 microinches. When hard disk crash happens, the read-write head will literally crash on the platter and with it physical bits of the platter. Data is literally, physically scraped off.