See? That's what I'm tellin' ya. Since we don't know which it is, DPST's the easiest way to go. But if you *really* want to use the switch you already have, you can probably figure out with a multimeter/voltmeter what's going on. You'll have to have your computer running, open, and doing something that'll provide a lot of activity to your HDD--defrag, maybe. First, put the positive lead of your MM on the pin on the mobo where you attach the positive lead to the HDD LED. Then put the ground lead on your MM to any ground in your case. The metal casing of the PSU should work, as should any black wire on the PSU. If you get a steady voltage (I'd guess it'd be 3.3), then you know that the power is constant and that the ground changes. If the voltage fluctuates, then it's probably the other way around. If you want to make absolutely sure, switch it up and get some power from somewhere--I'd prolly use 5v off of a molex (red wire); I don't believe it'd be a problem to run more than 3.3v through it, though I'm not absolutely certain--and then run the ground lead on your MM to the ground pin for the HDD LED on your mobo. Then it's the other way around--if you get fluctuations, then it's power that controls the LED and not ground; if you get a steady voltage, it's the ground that changes. If you get fluctuations both ways, then either a) you've accidentally done something wrong, which is likely (not just you, I have a dickens of a time trying to get my MM to do what I want it to) or b) both power and ground flip on and off, which is very unlikely. In that case, either common power *or* common ground should work just fine. I'd use common ground, m'self, b'cause then you don't need to worry about resistors and whatnot.
