Its going to be difficult to pinpoint the exact culprit without having extra parts to swap. From what you have mentioned, I'd say it could be, the power supply, motherboard, or your processor. Not saying that it cant happen, but I am unaware of any way the motherboard, processor present or not, can stop the hard drives from spinning up once the power supply is turned on. You could always disconnect the sata cable to verify this. It's possible the power supply is faulty being overloaded when the processor is present. But that takes us back to the three main components previously mentioned. Do you have another power supply you can try?