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Recently, I got another HD 5870 to crossfire with the original one in my machine. They are made by different manufacturers but everything else is the same. I tested each card individually and they both worked fine. Unfortunately, if I have both in the system at the same time, it won't post. I know the power supply and video cards are not the issue but i can't real

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looks like you question go cut off. anyway, have you checked your motherboard for an auxilary power connector somewhere near the PCIe slots? – Xantec Nov 21 '10 at 1:33
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if the mobo allows asymmetric crossfire, I can't imagine symmetric crossfire with different vendors being a problem...Xantec brings up a good point. It sounds like power/not seated to me. – RobotHumans Nov 21 '10 at 2:05
    
I checked and there is no auxilary power connector and I also made sure both cards were seated well and I still had problems – Ry03 Nov 21 '10 at 3:49

This is very, very likely to be a power supply related issue in my experience.

Two high end video cards draw a LOT of power:

enter image description here

Going from one 5870 to two means +170w under load, and +26w at idle.

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