Bitfury power requrements

[idecable]

A friend sold me a Bitfury 400g, using a Corsair RM850 power supply.

I have a question. Do I need to absolutely use all the 3 power sockets on the board to get all 16 cards working?

I was told to leave it as it is (using the two 6 pin connectors only, not using the 12v at the other end of the m-card).

(I'm using c-scape m-card v3).

I replaced 4 cards lately. I can get them all detected one by one, but when I put them all, the last one (F) does not get detected..

My question is, do I depend on the 12v rail if I want to run all the 16 cards properly?