Topic: Problem getting 5-6 Nvidia cards seen on 990FXA-UD5 (Read 505 times)

I'm getting a very frustrating problem that I'm hoping can be sorted.

I've got the following setup, and I've been spending the last couple of weeks trying to get this to work properly:

Gigabyte 990FXA-UD5 rev 3.0 motherboard;
Corsair 8Gb memory
Sempron 145
Corsair RM1000 PSU
and 6 x Gigabyte nVidia 750ti Windforce cards.

I've not been able to reliably get 3 of the cards to work on this board. I can just about get a 4th working on a powered PCI 1x riser, but after a while get what sounds like a crackling noise - not sure if this is the riser or just a fan noise. The card is powered using the 6 pin socket on top, so I'm not drawing power from the riser, but have tried using the molex connector on the riser.

I've got 6 of these risers, but noticed that they're a little too short. I can't mount the cards correctly if I use them.

As a result, I ordered 6 USB PCI risers, 60cm so I had a bit more flexibility in mounting the cards. I tried mounting 4 cards, the PC doesn't even post. Gradually reduced these down to 1 card, still nothing. Used both the power port on the riser and the 6 pin power port on the top of the card, still nothing. I've tried swapping round the USB risers in case I had a dodgy one, still nothing.

As I'm quite open in terms of OS, I've tried Xubuntu, KopiemTu (BAMT-like distribution for Cuda / nVidia), Windows 7 and Windows 8.1. Xubuntu / KopiemTu detect the cards but refuse to mine at all (haven't done too much diagnosis so don't know what's up). Windows 7 could cope with 3 cards before deciding it had enough, the 4th card showed an error 43. After trying 8.1, the 4th card is seen and mines. 3 of these cards are mounted on the board, the 4th using a powered riser.

Ideally I'd like to use all 6 cards, at the worst 5 (as I've got another PC I can mount the 6th card in). I'd like to use the USB risers if I can, as it gives me flexibility as to where to mount the cards. However, it's frustrating that I'm getting nothing at all from them.

Any ideas as to what my issue may be - is there a particular issue with the motherboard preventing me from using more than 4 cards?