When a disk drive stores data, it always reads or writes one complete sector. It is too difficult to align the read/write head at an arbitrary bit position when it is spinning so fast. If the read-write head were between bit positions when the electronics started to read the bits, the wrong values would be sensed. So the disk drive waits until it senses the dead space between sectors before it starts to read or write. It then continues, reading or writing all the bits in that sector. Many disk drives today store 512 bytes (or 0.5K) per sector.