Mac Pro won't boot past gray screen?

About a week ago my Mac Pro started freezing at boot up. After pushing the power button, the chime goes off, the grey screen appears with the spinning 'gear', but after about 5 seconds everything freezes. There is no way to reach the login screen and I have had to resort to forcing shut down.

The computer even freezes when I try and boot from the 10.5 disk that came with the computer, as well as with my retail copy of Snow Leopard. Therefore, I've tried booting the Mac Pro via target disk mode, and tried to repair the disk using Disk Utility on my MacBook Pro, but this didn't have any effect.

Before I haul it in to the shop, if anyone had any suggestions or advice it would be greatly appreciated.

Thanks!

GP

While this could be a dead hard drive or other hardware failure, it does also sound a bit like an issue I had with a bluetooth headset while it was charging. Apparently it was defective, and the Mac was trying to boot from it. So... the first thing to try is unplugging EVERYTHING except a keyboard and mouse. Heck... try it without a keyboard and mouse and see what happens. If you have a monitor or keyboard that doubles as a USB hub, unplug everything out of those. Hopefully this will do the trick. Then it'll be just a matter of figuring out what the defective USB device was.

If that doesn't work... you said you booted the Mac Pro in target disk mode? Was that successful, as in the other Mac was able to read/write to that Mac Pro's drive?

Yeah, when the problems first started, I booted in target disk mode and was able to recover the data I needed from the Mac Pro's Mac OS hard drive.

The weird thing is that it won't even boot from the DVD, it still freezes, so this makes me think that it is a hardware issue, but any other advice would be greatly appreciated.

Thanks!

I am having the same problem!
Also on Mac Pro quad core 2.8 with 12GB ram.
Have you found out what is causing this?