Superpilot, there is a minor mistake in your assumption. The HD will start spinning even without any data cable connection, just plug the power cable in and start the machine up...
Mike, go as all others suggested, remove all and sundry, re-seat the CPU and than install step by step with start-ups inbetween.. .One more thing, remove also all RAM and while you have a bare motherboard, take a soft brush and clean it from any dust..... .Now check also (very important advice!!!!!!!!!!!!!!) the slots you put your cards in, you are not the first one who may have accidentally bent one of these tiny contacts thus producing a shortage. And, use a magnifier....