Preciseley.
You are actually potentially damaging your cards by not having all the
risers on the main PSU. You should connect all devices used in the
system to the main PSU and/or also ground both PSUs to the same
potential.
Well, the common denominator seems to be using two PSUās. Which iād find a bit sketchy.
If one psu powers starts up slightly before the other, then potentially you could feed power from lets say the molex connectors back into the board which may be the cause of this. Also regulation could be an issue as switchmodes regulate themselves by measuring the output voltage, so if you have a psu already powering the bus it could also cause issues.
I opted for a 1000w power supply, and Iām using powered PCI-E and molex connectors. No issues.
That is true.
Therefore it is important to avoid feedback by grounding both PSUs together and distributing the power from one PSU to the mainboard and all powered raisers at the same time. The second PSU can be used to just feed power to the 12V PCIE connectors of the graphic cards.
Even that Iād find sketchy. The problem here isnāt powering molex connectors and powered risers at the same time, the real problem is trying to cobble two PSUās together which I donāt think is a good idea.
As I just said if one psu powers slightly before the other.
Letās say psu1 powers up slightly before psu2. The power lines on psu2 are effectively at ground through the smoothing capacitors so quite a bit current would be drawn via anything connected to those lines.
The correct solution would be to use a PSU rated for the load required.
You will not need two power supplies using Rx480 cards they are very efficient and you will power all 6 cards of one supply,two power supplies was only necessary using older gen cards that were power hungry,like any hardware faults can occur especially with risers but you will do no damage using both powered risers and plugging in motherboard molex connection as well,your rig could be unstable if used without motherboard extra power and when u receive your board there is a warning on sticker not to forget to plug them in,I have 2 rigs with 5 cards each running none stop for a year with no problems and I researched this on bitcoin foram before choosing USB POWERED RISERS,donāt listen to scaremongering from ill informed forum users only people with real world experience and using this setup can advise,if this was an issue these boards would clearly state do not use powered risers but there is no warning because as others have pointed out it is correct way to go.
Depends on the PSU.
I have 2 750 Watt PSUs.
Each of my RX470 draws about 100 Watts just on the core. + PCB + Mainboard and CPU.
So 1000watts is about minimum I have to concider.
Im still confused if I should plug it in or not to do keep them plugged in or not, sounds like more are say to keep everything plugged in. Iāll do some more research, before I start plugging everything in, it does seem to be issues when two PSUs are hooked up. Also it does make sense that it should only use power that it needs.
As I said, the problem, isnāt using both connectors at the same time. The problem is from using multiple PSUās connected to different parts of the system and not powering up at exactly the same time (which is pretty impossible to do as electronic components always have inherent differences). If you are only using 1 psu (Iām running a 1000w with 6x rx470ās and Iām using both powered risers and the molex connectors) then you shouldnāt have any problems.
Itās actually the correct way to do it, as you spread the power evenly to the graphics cards instead of too much power being drawn from the molex or the PCI-E connector on the card.
If I were to use two PSUās myself (which I probably wouldnāt) the safest way would be to link the power lines together between each PSU before they hit the system (ie 12v to 12v, Gnd to GND, 5v to 5v etc).
Have a look at this:
My PSUs start up at the same time due to the spliced PS_ON# lead.
As the 2 PSUs are exactly the same, I do not care about different starting times.
All molex connectors of the raisers go to PSU1, which is connected to the mainboard, the extra 2 molex on the mainboard, CPU power, and 3 of the RX470.
PSU is stripped down to just the PCIE leads + PS_ON# and power the last 3 RX470. Ans sometimes there was like 2 or 3 seconds between switching on the 2nd PSU. And nothing burns. 
Before I hat the PS_ON# spliced, I just turned on the PSUs with the switch on the backside.
So for me this is working. If you are in doubt, go for 1 single PSU with 1000 watts. (1200 watt to hit the load sweetspot)
This stuff has been covered and argued about already SO MANY times. Go to the ethereum mining forum at https://forum.ethereum.org/categories/mining and do a search for that motherboard and you will see all the same arguments.Those extra molex input on the Asrock pro-BTC board are really mostly useful when youāre running 5 or 6 GPUs and then the PCIE bus is struggling to deliver enough power. Someone earlier mentioned ānoobsā using powered USB risers, etc. I have been mining ETH with 2 of those motherboards with 5 GPUs on each, and I have the motherboard molex sockets filled, and I use ALL powered USB risers (even in the x16 slot). Itās been fine for about 8 months running 24/7.
Ever since the Litecoin GPU mining days of 2014 people have used 2 power supplies and it works fine. You just have to know how to start the second (GPU only) PSU before the primary (motherboard + 1st GPUs) PSU. They donāt have to be started at the same time.
As was also stated - the MOST important thing about using dual PSUs is that the PSU that powers the motherboard, CPU, 1st (and 2nd if you want) GPUs must be used to power ALL the risers. The 2nd PSU is only used for the 6/8 pin power to the additional GPUs. You can buy cheap little relay boards which will power both PSUs on together if you want but itās easy enough to just short the green start wire pin to ground in the ATX plug of the second PSU and just turn it on before you trun the system on.
Ok, thanks again guys I only have one 1000w power supply, so I will hook them all up, I appreciate the responses.
I use powered risers without the molex connectors. 3x Rx 470 and 1x Rx 480 on 1 board.
Well this should help you,pop over to Claymore thread at bitcoin talk and in last couple of pages you will see a pic of a burned out motherboard power connection caused by using none powered risers,motherboard pulled to much power through the board and wires could not cope,this is what can happen if you are not setup correctly,if he was using pro bitcoin board with extra power connections he would have distributed the power more evenly and better still using powered risers would have helped,he was told by experienced users to use molex power and powered risers,just because users here are getting away with not using correct methods for power with this board and getting away with it donāt follow there example if you want to stay safe,lesson for everyone there,i have posted this here in hope others will find this useful if not sure what to do.
Thanks, I am currently mining with the molex power hooked up and power risers.
isnāt it better to use the powered riser cables? i myself use this bord with 6x rx580 with powered riser cables and ofcourse power to the cpu. i was worried about the motherbord burning out with 6 risers over that one molex port.