Well it looks like I killed 2 magnetic drives and 2 SSDs. Luckily for me the one drive I hadn't backed up was spared, a WD 5TB drive loaded with pictures.
My condolences. That really sucks.
Why the %#@* the manufacturers can't settle on standard pinouts, at least for SATA power is beyond me.
Heck, back in the day, Dell even used a non-standard ATX motherboard power connector pinout, with potentially destructive consequences!
An old article from those days which describes the issue: http://www.informit.com/articles/article.aspx?p=339053
Obligatory XKCD link: https://xkcd.com/927/
one question: With my old Corsair HX850 I added a Y-splitter to the EPS 8-pin cable and an extension as well (my board has 2 EPS connections). EVGA supplied 2 EPS cables but they're too short to run them the way I want. I don't dare use that old extension/Y-splitter before I know for sure whether it's safe. Anyway I'd prefer to buy new extensions as I only have 1 now. I see them for sale all over the web...is there any danger they'll fry my CPU?
Extensions and splitters (* see below) should generally be pretty safe, since they're intended to pass the existing pinout through unchanged. Mis-wired ones do happen occasionally though, especially if you're ordering stuff from random third-party Amazon sellers or eBay. It is best to at least do a visual inspection to verify that things appear to be wired "straight through", or (if you're more paranoid) invest in a cheap DVM to allow you to ohm out all of the connections to check for correct wiring.
* There's another potential issue with splitters though: If the power draw from the device(s) the splitter is feeding exceeds the rating of the contacts in the upstream end of the splitter, the connector can overheat. Overheating in turn damages the contact surfaces in the connector, resulting in further resistive heating (i.e. a feedback loop), which can eventually result in the destruction of the connector itself, or even (if you're extremely unlucky...) a fire. Splitters should only be used if you know the current carrying capacity of the upstream (pre-split) connector won't be exceeded. For lower-power devices like HDDs and SSDs, I generally don't worry about using power splitters. But for higher power devices like a motherboard or GPU, splitters can be a problem.