I'm pretty sure that's a myth that keeps getting re-hashed(for some reason) from the ribbon powered riser days..
(this coming from a guy that just murdered 2 cards tho.)
Usb powered risers seem to be totally independent. I've been running dual psu's for months in pretty much any fashion without issue(until now.) even non-grounded together.

Case in point: When said idiot forgot to turn off the second rig/psu before uplugging the pcie and molex from his poor 480.. that psu was powering both riser and gpu. it was also powering 1 570 the same way.. all 6 570's survived. The massive back surge when unplugging the pcie connector is (most likely) what took out the 470 that was plugged into the motherboard slot on the second rig.. yet the 480 + 470 on risers survived.(slot still works fine)

I dunno, just ordered a server psu to play with.. maybe I'll end up frying all my cards/burning the house down :/

Had 1 hx850 powering 5gpu+5 risers.. second powering 2gpu +2risers + 3 cards idling @ bios screen in the second rig..

I'm leaning towards the true cause of death may or may not have been related to some *Idiot* forgetting to turn off the second psu/rig before unplugging pcie and molex connectors....
(dang psu's that run fan-less until super hot anyway.)

Well, the good news is I got 7 570's working in windows 7 with the mod! It was a simple power problem..

The bad news is, I thought it would be a 'great' idea to test the power theory by using 1 pcie chain + 1 molex chain from my 4 card rig that was using an ancient DFI lan party mobo.. + hx850i with that pc idling in "bios screen mode"..
First I tried 1 480 + 6 570's that worked fine.. then tried all 7 570's (powering 2 risers/cards with second psu..) worked fine.. both with multiple reboots to make sure it was working for real.

All was good until trying to hook the 4 card rig back up.. bios beeps.. thought it was BS'ing me at first but, guess not .. power color red dragon 480 + a 470 that was minding it's own dam business in the motherboard slot -- both apparently dead.(tried in 3 pc's. fans don't even spin.. nothing in device manager/atiflash..)

FML! I guess. 1 480 & 470 survived at least.. grr.
Glad it says newbie under my nick some times..

dang, 7 570's show up in win 7 with 0 error codes on a msi gaming 5 mobo but, only 6 work :/
with 7 plugged in, when starting mining software lan cuts out, stuff starts crashing/freezing/time to reboot.
 

guess I'm doomed to linux?.. (because, #$&% win10-anything.)
funny, I thought I'd be running 8 gpu's on this mobo..(lawl)
sigh.

Did you have much luck with the samsung card?

I just got my first armor card. had my fingers crossed for samsung.. which I got but, not too sure so far, 990mv just to get it to stop throwing hw errors at 2040mem 1750strap..
Have a feeling it's gonna drive me nuts like my first hynix card did 

nm.. still slow errors, bah.

pushing 3x 470 nitro's a bit hard at 230~ sol each.. + overclocked 2500k cpu + some hdds = 620-650w@wall..

630w Rosewill green series psu... been perfectly stable since 9.0 release..
guess it doesn't have any "fancy protection" other than when it catches on fire and/or explodes 
(definitely seeing the spikes to 250+w~ per card though.. rather scary.)

Anyone else ready to take their mining rigs into the bathtub after reading every single page and post of this thread? thanks..