Power coming FROM external HDD (USB) to MB causing problem?

[Scope]

I have had a number of problems with my machine recently, and was wondering if my recent discovery could have anything to do with it.. It seems when I plug my external HDD into the USB port the machine will stop half way through the POST.. Take it out and it will work just fine.

I noticed the USB lead coming FROM the HDD (which has its own PSU) has electricity coming through it, could this be causing the motherboard from booting? I know USB has 5v coming from the MB to the HDD, but the other way around?

So, is this normal, and could it cause the problem I am having?

 

[Akempster]

some PSU's will fail to start the PC if more than one PSU is used, you may need a psu power adapter to trick the PSU in believing its powering the mobo as well as the other components