Recently, I was screwed by my big box of cables, when I pulled out a SATA power connector for my modular power supply, and learned very quickly that though the ports universally fit across modular PSUs, the pinouts are not actually intercompatible, and you can and will fry your hard drives. I didn't know this; I had foolishly assumed that SATA power connectors were standardized on both ends.
6 pin EPS and 6 pin PCIe power cables are not the same. Even though the connector may fit they are not the same pinout.