Before understand this you will need to clear the concept of tracks and sector.

                          fig 1

In fig 1, red color shows a track , purple color shows disk sector and cyan color shows track sector.

Seek time means  the amount of time it takes a hard drive’s  read/write head to find the physical location of a piece of data on the  disk.

Latency means the average time for the sector being accessed to  rotate into position under a head, after a completed seek.

QUESTION:

Consider a disk with 16384 bytes per track having a rotation time of 16 msec and average seek time of 40 msec. What is the time in msec to read a block of 1024 bytes from this disk?

(A) 57 msec      (B) 49 msec

(C) 48 msec      (D) 17 msec

Explanation:

Time in msec to read a block of 1024 bytes (Access time or Disk Latency) = seek time + average rotational delay + transfer time.

If there are 16384 bytes per track there are 1024/16384 tracks to be read for this block.

Seek time = 40 msec

Rotational delay = 16 msec

Transfer time    = (sectors_read/sectors per rev.) x rotational delay

= (1024/16384) x 16 = 1

average rotational delay = rotational delay/2 = 16/2 = 8

access time       = 40 + 8 + 1 = 49 msec