I recently purchased a flashed to 5.1 2009 3.46 twelve core Mac Pro, 16 GB 1333MHz DDR3 ECC and a 2GB Hitachi HDD. Initially the Hitachi HHD heads were constantly working while the drive was idle but I don't notice that now. The issue seemed to end after I hooked up a FW 800 drive for a port test. I have upgraded to Yosemite and finally got Apple Hardware Test working. I have run AHT a few times, both the short and the extended test and keep getting error code 4M0T/1/40000002CI-0. From what I have read this is a possible PCIe fan or heat sensor issue. I zapped the PRAM but it made no difference. I haven't noticed any issues with fan noise or kernel panics and such. It's a quiet machine up here on my table. Just quiet fan noise and normal HDD sound. Slot 1 has a ATI Radeon 5770 card and the rest of the PCIe slots are empty. Is there any way to easily further test this? Could it be a non-issue or it possibly could be corrected with third part software if it does become an issue? Should I contact the seller and return it ASAP? The seller only warranties it for 2 weeks.