Data is organized on the disk into tracks and sectors. Each track has a unique number. First track always has the number 000. It is known as track zero. Similarly sectors on a track are also numbered. When operating system wants to read data on the disk, it specfies the adress of the location. By using the provided address the disk controller moves the read/write heads to the required track. It also uses the motor in the disk drive to rotate the disk platters.The time used to move the read/write head to a particular track is called seek time. Once a track is selected, the disk controller waits untill the read/write head reaches the required sector.The time used to reach the head at the begning of a required sector is called rotational delay. When the required sector comes under the read/write head, it reads the data from the disk . It then sends this data to the processor. The time consumed in this process is called the transfer delay.
So the access time is Calculated as:
Access time = Seek time + Rotational delay + Transfer delay It is clear that the seek time and rotational delay involve mechanical parts. So the hard disk is very slow as compared to CPU.