For one your HDD has a cache which depending on the setup should store most frequently used HDD addresses, or the most recently used HDD addresses . . flash cards instead of a HDD is definately the way forward, but thats a massive amount of achitecture change in one go. Plus a certain amout of your computer's core operation is set around the fact data access is slow from a HDD.
And did you know, though the Ipod started with a spining HDD, the newer ones have flash in them? . . . so my lecturer recently pointed out, though i trust him less since his half hour rant about greek architecture.