Thank you for all the answers !
I finally get it running with the 6 GPU's after 2 days of trying !!
I'm not really sure about what exactly made it work well .. but it's working !
If it can help somebody, that's what I did :
I took out all my GPU's, reset my BIOS to default settings, uninstall the drivers. Then I use a VGA cable to my monitor, go to the BIOS and change the option that allows more then 4 GPU's on my Mobo (4G), and install the drivers for my graphic cards (still with no graphic card plugged in).
I added then one graphic card, turn on my computer, let him load it, and do one more restart before lauching Ethminer and look if everything is okay. I did that for each graphic card one by one, and it finally worked well !
I also overclocked them as I had them before, but I didn't do that for each card individually.
I got one GPU at 62-63° and one at 72-73° , that's a big difference, and they having the same hashrate !
I didn't push them to much, I'm dualmining around 21,5Mh/s ETH and around 210 Mh/s SIA .
I was wondering, should I overclock them individually ? And can I do that with MSI afterburner?
And another question I had, I'm mining with Nanopool , and they are telling me I get 0,7 ETH per month with 139 Mh/s average in the last 6 hours. But if I look for example on Cyrptocompare they are saying 0,99 ETH per month with 139Mh/s !! This makes a big difference !
I know the difficulty had a jump just a few days ago, so maybe Cryptocompare didn't refresh the difficulty yet ?
Or Nanopool are paying 30% less then the real rate ? If that's the case, I should move to another pool !
I also got one GPU that's makes much more noise then the others, and I can also feel it's having much stronger vibrations then the other ones when I touch it. Should I send it back to get a new one ?
Thank you !!