I've often wondered why spinning hard drives don't have multiple heads per platter. For example, two heads spaced out on the same arm could conceivably cut seek times in half. Or, a second arm on the opposite side could offer similar benefits.

The head-positioning firmware would get more complicated, yes, but for the fixed cost of getting those algorithms right many multiples of increase in seek times and transfer rates would be possible, by adding arms and heads.

Perhaps the competition from SSDs will trigger some new approaches in mechanical drives, as well.