In order to fully understand a hard drive you have to know how one works physically. Basically, there are discs, one on top of the other spaced a few millimetres apart. These discs are called platters. Polished to a high mirror shine and incredibly smooth they can hold vast amounts of data.
Next we have the arm. This writes and reads data onto the disc. It stretches out over the platter and moves over it from centre to edge reading and writing data to the platter through its tiny heads which hover just over the platter. The arm, on the average domestic drives can oscillate around 50 times per second. On many high-spec machines and those used for complex calculations this figure can rise into the thousands.