I'm on a used Mac Pro 3.1 (early 2008) here that came with no OS pre-installed. Then did the following steps: https://discussions.apple.com/thread/5182238 which I'm not getting any reply so far at all over at Mac Discussions, resulting in a working Snow Leopard and Windows 7 64-bit for 12 hours, and then suddenly no bootup anymore whatsoever. Not on Windows partition, not on Windows install DVD, not on Snow Leopard Partition, not on Snow Leopard install DVD. For error messages/behaviors see link. Should probably also mention that by now I've also tried resetting both SMC *AND* PRAM, no change. Tried safe boot, all that happens is a little progress bar on the white booting background, then before it gets to 50%, it's suddenly replaced by the spinning clock-like thing that then just freezes in unresponsiveness just like when trying to boot from SL partition or SL install DVD. By now, I'm desperate enough to try the FireWire target mode suggested in your guide: http://guides.macrumors.com/Mac_doesn't_boot , using my old but still working Mac Pro 1.1 (mid-2006) with Tiger and Windows XP Home 32-bit on it. Oh joy, with only a single monitor and keyboard for both Mac Pros!