Mac Pro 2008 Refuses to Boot
I ended up solving this. I had installed a SSD, and connected it to the hidden sata port. It worked for a couple of months. Then one day I had this issue. I took the ssd out and installed OSX on the first drive bay and it booted! I'm guessing the fan chassis must have been snug against the connector or otherwise some dust was causing grief. In any case. I'm glad I didn't pay an exorbitant fee for the rare aftermarket PSUs.