Hi Thanks for your feedback.
We have two leds on the board and the both led behaiver is the same as the Power LED(blue is system for ATX; green is power for PC) , we didi't have HDD LED on the ATX board.
There is a Optocoupler on the ATX board, and inside the Optocoupler there is a LED; so if you plug the right polarity the power LED will work good. otherwise the led won't work.
The power LED on my PC case does not work releated to the Current driving capability of the motherboard. In your correctly connect situation. the Power LED signal will driving two leds. so you can't see the brightness of the case power led; (on my side when connect in the right polarity the case LED brightness will decrese so does the current)
when you swap polarity of both side. the led in the Optocoupler in the wrong polarity; the Power LED signal will driving only one led. and the case led works ok.