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The risers are powered by 2 different sata chain cables coming from the main PSU. The thought of running all Risers and 3 GPU's from the Slave PSU and the other GPU's and the mobo from the main PSU is interesting.

I see many people recommending not using SATA to power the risers at all and that running multiple is a potential fire hazard. See: https://www.reddit.com/r/gpumining/comments/6pqb8i/question_about_safe_power_draw_for_pcie_risers/

If your running 5 risers on 2 chains that might be an issue since SATA cant provide enough power per chain. If you can run the risers directly on molex from psu i think that would be preferable.

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I thought I read somewhere that since the risers are extensions of the motherboard it may be safer to run them from the same PSU that powers the motherboard. I've also read not to cross the power like shubaduba suggested.

Yes that is probably wise to avoid having different electrical sources per component. I think shubaduba's suggestion running two cards directly on the motherboard might be a solid idea. Have 3x gpu and 3x risers completely powered by slave.

Thats all ive got, good luck!