Why does the risers molex have to be on the same PSU than the mainboard?
Problems can sometimes arise if you don't. The mobo draws that power normally without risers, so when you use risers it is suitable to continue that task for the mobo psu.
I just want to make sure since I don't see the logic there.. the risers have to be connected to same PSU than the mobo, but it can be different PSU that the one the GPU are connected? Is it absolutely necessary? What have to be taken care of if you can't?