Almost every gate exam will have a question on hard disk addressing scheme which is very easy to understand if we know the physical structure of hard disk. You may have to look into this few times to get a good understanding, so don't set back if you were confused by looking into this initially. consider the below picture
consider the top view of the hard disk.Hard disk will have multiple number of cylinders and as shown in the picture each cylinder will be placed one after the other in a row from inner circle to the outer circle. each cylinder will have multiple number of platters placed from top to bottom as shown in the right part of the picture. Each platter again will have two surfaces - surface 1 and surface 2 as shown. Each surface is divided into equal number of smaller parts called sectors.
so the division goes like this
cylinders - platters - surfaces/tracks - sectors. Sector is the smallest unit of division.
Addressing will be done in the sector level (Each sector will have an individual address). Each sector will be given an address like <cylinder no.,surface no.,sector no.>
consider bottom part of the picture which shows one surface divided into 8 sectors - usually this is called as a track. Assuming the numbering of cylinders, surfaces,sectors start from zero, the first sector (present in the first cylinders first surface) will be addressed as <0,0,0>. As here each track is divided into 8 sectors, each sector address can be seen in the picture marked from <0,0,0> to <0,0,7>
Following this addressing continues to surface2 of the first platter. there the addresses will be from <0,1,0> to <0,1,7>
Like this all the surfaces/tracks of all the platters(here in the picture we have 3 platters and each platter has 2 surfaces) in the first cylinder will be addressed and then the addressing continues to the immediate outer cylinder that is cylinder 2. This pattern of addressing continues until all the cylinders are completed.