This has to be done in order to be nothing is amiss. Small things can be forgotten or taken for granted. I would under these circumstances return the bad one and just leave it at that. The longer you play with it may cause further problem(s), then they can blame you. use the warranty and state one works and the other doesn't under the same test bed.
I recently received two, ECS GOAL3+ MOBO w/CPU (AMD Sempron 3000+).
I set them up following step by step procedure for bench checking...
The first MOBO works fine. The second, the CPU Fan starts but nothing else.
I used the same power supply, memory, keyboard, monitor, and mouse in each mobo. Why did the second fail to function correctly? Is it a bad CPU? or could the CMOS Battery be bad on the second mobo?