I have a 1,1 Mac Pro that is stock with 6gb ram. I installed Mavericks on it and was running for a couple days just fine. Installed a ati 5780 and again ran just fine for a couple of days. Then one day it just wouldn't wake up. I could tell it was doing something, as the TV would stop displaying "no signal" but it would still be black. I removed the 5870 and put the original video card in so I could get boot screens. It was just booting to a flashing folder. From what I read, it means it can't find the startup disk. I boot with option key, and select the harddrive (shows the system partition and the recovery) but then it just boots to a grey screen. I tried resetting PRAM/NVRAM, leaving everything unplugged to reset SMC, and starting in safe mode, and nothing works. I hold shift and I just get grey screen, no folder. If I hold shift after selecting the drive from the option-key menu, I just get a grey screen as well. The other stuff I read indicated the drive might be bad, so I boot the mac pro in target disk mode, and booted from that on my macbook pro. It seemed like forever, but eventually got to a screen saying installing os x, once it went through everything, it booted to my desktop, and everything was normal. It even brought up chrome and the youtube video playlist I left off on. Settings, data, everything is fine on the drive. Not sure why it was installing os x or what that was about, but it's definitely bootable. So, any idea what I can check next? I just bought RAM, processors, and this graphics card to update it, so I'm hoping its not a hardware failure that costs too much to fix.