Which surface of a hard drive needs the most coolling?

I would cool the top of the drive (with the label) because that is the part of the drive that I always notice is the hottest. I don't imagine that there is much current going through the PCB so it shouldn't need it's own cooling.

Depending on how the drive looks on the bottom, I suspect an argument could be made. If the PCB is small and you can see most of the circle where the spindle is. The underside of the hard drives has grooves that is increasing the surface area to cool, just like a heatsink. Therefore, it could be more efficient to cool the underside for more heat dissipation.

Damn. Got here too late. Spot on as alway wusy

An HDD is a rectangular prism, thus it has 6 sides... not 4... so you mean... all 5 sides except connector area =)

I would say between top and bottom, not much of a difference, it'll cool well both ways. HDD's are pretty much all metal anyways, so cooling one part will cool the rest. Optimally though is to find a way to attach a cooling fan (preferably 120mm) onto the racks for the HDD's, cooling them all both above and below with better airflow.

I do that with my 6 external HDD"s and my desk fan.

Lets see... if we say the platter of the HDD is 3.5" diamater, and it rotates at 7200RPM, then we can safely say its

7200 * 3.5" = 25,200 inches/minute

25,200 * 60 (min per hour) = 1512000 inches/hours

1512000 / 12 = 126000 feet/hours

126000 / 5280 = 23.7 MPH

Now, lets say that gas is currently 2.15 a gallon for the plus variety (we feed our HDD's decent gas). It's also around 12c a kWh here, so if we do a quick conversion:

2.15 / .12 = 17.92 kWh

Now, an average hdd uses around 10W, so:

17.92kWh / 10W = 1792 hours.

23.7 * 1792 = 42470.4 miles.

Therefore we can say as of now, a HDD gets 42470.4 MPG. Of course, real life mileage may very due to the speed you drive, air resistance and errors in my calculation.