The Mac Pro here (2008 model) started acting up as of yesterday during booting - when I turn it on, I hear the typical sound, the Apple logo appears, it disappears.. and the monitor remains in that grey tone.. forever - nothing else is happening. This machine has Snow Leopard installed. When I try to e.g. boot from my Lion USB Recovery HD, i plug in the USB stick, press alt during bootup, both systems (the SL one on the disk & the Recovery on the USB stick) appear, I select the Recovery HD.. and again, the same (logo, then grey forever).
What's going on here and what can I do?
following the numerous suggestions below (thanks!), I checked the verbose boot logging, but nothing too troubling appeared, well sort of. Right before the monitor turns grey again, the color of the logging font turns from white to pink?
.. (the image is kinda off, somehow it doesn't capture the pinkishness of the color) then grey forever again.
Additionally I could enter safe mode, but the same is here.. colors seem strange & not entirely correct (I suppose it's not intended that way, right?):
I put the Nvidia 8800GT in the other slot on the mainboard, the same again. Is this a sign of a failing graphic card (the boot manager menu looks just fine)?