How does the system know how to read the correct block from a disk?
Finding the right track is relatively easy. There is a motor which can be moved a precise distance. Then the head can measure magnetism to the right and the left and use this to move to the precise centre of the track. 

The sectors all have a header. The read head passes sectors at high  speed. The start of the header has a distinct pattern that cannot be found inside a sector. So the read head can wait until it finds the start of a sector. The header also contains a number identifying the sector so the read head knows when it reaches the correct sector and can start reading. 

In practice, all sectors on the track will be read in case another sector is needed soon after.