Q. Describe why SSTF scheduling tends to favour middle cylinders over the innermost and outermost cylinders.

Answer: The middle of the disk is the location having the smallest average distance to all other tracks. Therefore the disk head tends to move away from the edges of the disk. Here is one more way to think of it. The present location of the head divides the cylinders into two groups. If the head isn't in the centre of the disk and a new request arrives the new request is more probable to be in the group that includes the centre of the disk therefore the head is more likely to move in that direction.