Are you sure your motherboard and chipset can handle it? Some disable pci-e slots when others are used. The fans would spin up since it gets power, but it will never be addressed by an OS due to hardware limitations of the motherboard.
But, I never got more than 4 280x cards working in windows. I went straight to linux. I use intel ivy bridge celerons though, and z77 chipsets...
Thanks for your reply, pontiacg5. I contacted Gigabyte technical support and while they couldn't give me any specific help for this problem as they weren't familiar with the ins and outs of mining, they did seem to indicate that the 1x slot could be used concurrently with the 4 x 16x slots. However, the mb manual makes no mention of this either way.
However to clarify, I have tried pulling out all other GPU's, and using just the 1x slot (so there would be no slot conflict) and I still can't see the GPU in that slot. Have tried switching riser cables, trying powered and unpowered, jumpered and unjumpered, and have tried this on two different rigs.
Anyway, any other suggestions on this would be greatly appreciated.