Thank you for the detailed description of the theory behind your methodology, jtr1962; I'm glad to see that you seem to understand thermodynamics a lot better than most, and that kind of approach gives me more confidence in the validity of the Test Bench results as a whole.
A couple of additional comments, for those who have trouble equating the similarity between power dissipation and heat generated: First, keep in mind that ALL power that goes into the hard drive becomes heat. Technically, a very small amount becomes mechanical energy, but since that's dissipated frictionally anyway, in the end it is ALL heat.
Second, as pointed out, since hard drives are externally pretty much the same--the shape and materials used are close enough to be functionally identical--the only thing that could cause a difference in how much heat is effectively "in" the drive would be interal layout, which the SR crew have no way of telling anything meaningful about (even if they tore the thing apart, they still probably woudn't be able to learn anything meaningful).
You can do this set of thought experiments to get an idea for why top-plate measurements aren't particularly useful:
1) One can assume that it's usually actuators, motors, bearings, and chips that fail due to heat. All of these are located inside of the drive or on the bottom surface. Let's say that, hypothetically, a drive manufacturer figured some magical way to transfer all the heat generated by the drive to the top plate, where it's away from the electronic components that it might damage. The drive would, based on this measurement, "look" incredibly hot, despite being very cool where it counts.
2) Similarly, pretend you had two drives, one with perfect thermal transfer (pretend it's solid diamond) while the other completely isolates the guts from the outside of the case with a layer of styrofoam. The styrofoam-encased drive would appear incredibly cool on the outside, since almost all the heat generated would be held internally, even though it'd rapidly bake itself to death, while the cool drive would appear comparitively warm on the outside surface.
As such, it's quite possible for an apparently cooler drive to actually be running hotter where it counts--in fact, if you give me two drives with the same power dissipation, I'd wager the one that felt hotter is the better design, as it probaly does a better job of dumping the heat to the surface where it can be carried away.
None of this, however, takes into account how good a job the drive does internally of keeping the important components cool, which leads to...
3) The only really good way to experimentally measure how heat would affect a drive would be to take a thermal scan of the chips on the bottom and, ideally, also the internal components. That strikes me as well out of the range of any realistic test SR could do, and in any case few of these components are near the top plate where it's easy to take a reading.
Hey, I could be full of it here, but it jibes with what I remember from thermodynamics.