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Windows 7 can be very difficult when it comes to going over 4 GPUs. Some people have had success with 5-6, but it's often hit and miss, especially when the drivers for these more recent cards don't support the old 6x GPU mod for Windows.
If you're not a Linux user, I would recommend using Windows 10, or at least Windows 8.1.
cheers I'll try on windows 10 later