Mac Pro stuck on spinning grey apple on startup
I have a Mac Pro that is stuck at the grey startup screen. Grey apple, spinning wheel. I let it spin for about half an hour and then went on to try other things. Every attempt still leaves me stuck at the spinning grey wheel.
So far I have tried:
Shut down, disconnected all cables, restarted with nothing connected. No change.
Reset PRAM to 3 chimes. No change.
Opened machine, removed RAM (3 1GB sticks) and HDs (2 1TB drives), replaced them. No change.
Tried booting from Mac Pro original CD. Nope.
Tried booting from Snow Leopard install CD. Nada.
Tried booting from Applications CD - was able to run Apple Hardware Test - no problems found.
Any ideas? The Mac is a Mac Pro 2.8 GHz Quad-core Xeon. Normally runs like a champ.
various, Mac OS X (10.5.6)