Calculating the Transfer Time for a Hard Disk
Sectors are usually 512B to 4KB. In your case, it seems that the actual time it takes to read the sectors is negligible compared to seeking (verify this!), and this could be what is meant by the following line:


  Since a sector is very small, ignore the time to rotate it through under the head.


To be sure, make the calculation with respect to the largest sector size (4KB), and check that the data transfer time is negligible. If it isn't, you are indeed missing a data point.