keep searching bro I am a bit busy but the key is
Modded 290x/290 drivers 7x GPU win 8 <<< Google and check out the litecoin link and custom installing them yourself.. I have 6 x 290x running on 1 mobo and I also tried 7 x GPU's and it also works but it was a temp test I am using win 8.
Went my custom built rack arrives ill have 2 x MOBOs 5 x PSU - 1 average x 600w(only cause it was free) just to power 1 x mobo and peripherals and 2 x 1000w Corsair GOLD. 2x MOBO wiill have 2 x 1200 GOLD CORSAIR I also have proper Add2psu switches not that cheap ATX cable splitter.
Win 8 Modded drivers 14 x 290x GPU's + 2 MOBOS - 1 x Big BANG MSI M77 & 1 x Fatality Killer Asrock Z87 Each handle 7 x GPUS and dont neeed High level CPU's
BTW I can confirm 2000 watt is fine for 6 x 290 ALl I have done is added --gpu-powertune -4 and its sitting on 2000-2010 but because I am using gold Corsair I am comfortable with that as 1000watt is not there max they always leave a buffer and 10-20 watt is WAY below there buffer. 2400w 100% will work with 7 x 290x
If no one believes me Ill post pics put it this way 1 x mobo ram, cpu & 3 290x = 1050-1070watt add another 1000w and you will be able to put another 3 no worries. Remember decrease powertune a touch
by followinf my google search tips this should DEF fix your issue:
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PS: we are talking about WIndows not allowing 6x GPUS right? (Code 43) issue, meaning Bios Detects them all? If Bios doesn't then its another ball game, you may need to shorten the Pins OR the Motherboard may not handle it Like the G45 cannot handle 7 x GPUs even though it has 7 PCIE slots. You need to research on the mother board. For example the bing bang MSI m77 MOBO will only handle 6xGPUs if you dont use a 3rd Gen CPU or above. With the 3rd GEN CPU it will handle 7 PCIE