External USB powered hdd turns off display

Hello,
this is a strange problem,

Whenever I plug in a Flash drive in my front panel USB ports it runs perfectly....but when I plug in the external 2.5 inch Usb powered hdd to the front panel USB ports...the display goes off and then a have to pull the cord to turn off the pc and restart it.

But when I plug it in rear ports it runs just fine....what can be the problem ????
My system runs on UPS APC 1.5Kv

Mobo : M4A89GTD PRO/USB 3
Cooler Master 690 II Advanced pc case
Sapphire Radeon HD5870
AMD X6 1090T CPU
Western Digital Passport 500 gb External drive
corsair TX 750W

I don't know the proper reasoning but the external USB always work with the rear port which is directly connected to the board and having the 2.0. I have connected my External Hard drive to the from USB and getting the same problem. After going through the some forum i come to know that External Hard Drive only works with the back Panel only.