When I first heard about how mechanical hard drives work, back in the days when the highest end 300 MB (yes MB) drives ran a stack of four or six platters, and some drives still used a sort of rack and pinion positioning setup rather than a voice coil, I assumed each platter would have its own actuator and heads were only paired up top and bottom per platter.

I was surprised to find that instead, the entire rack of them moved as a single unit, with so much energy wasted overcoming 3 or 4 times the necessary inertial mass for each seek. So now, 20 something years later, well after SSDs have come close to superceding spinning rust when all tradeoffs and variables are considered together, someone has finally gotten around to putting an extra actuator in these units. Great!

While on that subject I also never understood why the general layout of components inside these drives never iterated away from the very earliest RLL / MFM winchester mechanisms, with the spinning platter assembly slightly off-center in the case, and a single head assembly off in one corner.

I assumed at some point that the armature assembly would become miniaturized just enough to allow centering the platter spindle and adding another set of heads at the opposite corner of the case, so that each head set could address opposite sides of the platter, effectively halving access times even on single platter drives.