3 lessons
How I/O devices actually work, from the OS's point of view
What a disk really looks like mechanically, why I/O is agonizingly slow compared to the CPU, and the buffering trick that hides it.
Disk scheduling algorithms — choosing the order that hurts least
Given a pile of disk requests scattered across the platter, the concrete algorithms for minimizing how far the arm has to travel.
RAID and keeping data alive when disks fail
What happens when the disk itself breaks, and the clever math that lets you survive a failure without losing a single byte.