There is a better way and proved safe. My down clocked HD7970s are using similar power draw as HD7950s. No external power or 5.25" internal power needed. But OP's power arrangement is too dangerous, because both 6 pin together in one card should consume around 100-125 watts.
A better way is to spread to 2 SATA ports to protect the dual 6 pin circuits on board as in my arrangement for dual HD7970s.
Here is the link:
https://forums.macrumors.com/threads/1732849/
Thank you!
The total power rate of the Mac Pro PSU is 980w, Closing to 1000. The power of the SATA slot also comes from the PSU. Isn't that the same? Besides, for my chair here, the power seem to be stable and safe. I have used the CrossFire for half a year and nothing happened! Thanks for replying!
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