Have a magnetic disk and then read and write heads built on top of the disk area. Kind of like the design of a car wheel. Or like a circle with a dot in the middle and lines from the dot to the edge of the circle in every direction (theoretically). On those lines, will be all read and all write heads. One line is all read, next line is all write in theory. Or have marks on the line to read and another mark for write. So in the end, one theoretical line will hold all read and another line will hold all write. This way, no spot on the hard drive will be missed. One read head for every cylinder(a fixed point and the disk spins. That read head is dedicated to that spot on the hard drive as it spins-not sure if i am explaining it correctly). And the write head would be similar. There should be no moving parts except the magnetic disk inside.