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I'm running xubuntu on this motherboard and three sapphire 7950 graphics cards. The computer boots and works fine when I have just two of them installed, but when I attach the third the computer stops booting in the middle of the Xubuntu loading screen.

I don't think that this is being caused by lack of power because I'm using this 1600 Watt power supply and I'm able to navigate around the bios page just fine.

What could be causing this? Any suggestions on how to fix?

This is what I found when I go into advanced mode and look at the boot.log

enter image description here

and it keeps on giving me the message

udevd[340]: timeout: killing /sbin/modprobe -vb pci:XXXXXXXXXXXXXXXXXX [350]

When I do as khajvah suggests and enter advanced mode and then as root run sudo startx I get

Fatal server error:
could not create lock file in /tmp/.tX1-lock


xinit: giving up
xinit: unable to connect to X server: Connection refused
xinit: server error
xauth:  error in locking authority file /root/.Xauthority 

here is the whole message

enter image description here

share|improve this question
Are you certain that board supports three-way crossfire? Very few motherboards do. Additionally, what else are you running to need 1600W of power? If you can get past the BIOS, power is likely not the issue, as the system is stress-tested to some degree at that point. However, if it's not a certified PSU, you could run into issues. – Lyle Brown Oct 5 '13 at 22:20
If it was power related, the computer wouldn't pass the POST. Maybe it's a specific problem with xubuntu and multiple graphics support. – Doktoro Reichard Oct 5 '13 at 22:22
@LyleBrown the board has 6 ports for graphics cards and I know of others using it for this purpose. The power supply is only being used to power the three graphics cards, the hard drive and the mother board. – Loourr Oct 5 '13 at 22:27
@DoktoroReichard The only reason I'm curious of the PSU brand is because 1600W isn't very common, and cheap PSUs of that wattage can cause issues AFTER a POST. Unfortunately, I know this from experience. – Lyle Brown Oct 5 '13 at 22:34
@Loourr I strongly suggest you to post this question in ubuntu forums. If this is OS related, those guys will for sure help you. I really think this is OS, more specifically xorg related. – khajvah Oct 7 '13 at 21:23

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