Explain why SSTF scheduling tends to favor middle cylinders over the innermost and outermost cylinders.

The center of the disk is the location having the fewest average distance to all other tracks. Therefore the disk head tends to move away from the edges of the disk. Here is another way to think of it. The present location of the head divides the cylinders into two groups. If the head is not in the center of the disk and a latest request arrives, the new request is more likely to be in the group that contains the center of the disk; therefore, the head is more likely to move in that direction.