I recently bought a Seasonic X-Series SS-400FL to power my Gigabyte GA-A75-D3H motherboard.
To my dismay, I discovered that the included 8-pin CPU cable does not fit the 8-pin "ATX_12V" plug on the motherboard. Confused, I checked the manuals of other motherboards such as the Asus F1A75-V PRO and the Biostar TZ68A+. They all use the same type of plug and this Seasonic cable will not fit into any of them:
I thought this might be a rare manufacturing error, so I contacted Seasonic USA and got them to take a picture of a working 8-pin 12V cable. It is the same as mine:
The topmost connector fits into the "CPU" port on the back of the Seasonic, and I'm left with the bottommost connector with will not make an 8-pin connection with any motherboard that I'm aware of.
My question is: just what kind of cable is this? It doesn't seem to be standard EPS +12V, even though the Seasonic manual clearly states that it's EPS compliant.