Mac pro not booting up

Good Morning everyone.

I hope someone can help me with a problem i have. I have a Mac Pro 2.66 quad core with 8gb ram running mountain lion 10.8.2 . the problem is : if i shut the computer down, when i go to restart i get the system check sound then the white screen the apple logo comes up then i get the spinning cog. (all normal so far) the spinning cog after a while stops and disappears (still normal) and then nothing it stays on a white screen.The mouse moves around on the screen, but no full boot up screen stays blank.

If i plug in another bootable drive with lion on it. the system boots up ok in to mountain lion. i just then have to eject the lion boot disk. I have reset the pram, repaired disk permissions, and checked the hard drive all ok.

would appreciate if anybody has got any ideas what the problem is.

Thanks