I had exactly same problem.
So I tested boot by using a SATA cable from my desktop.
Connecting the cable with SSD and plugging another side to NUC board.
And SATA power cable was came from my desktop.
In this condition, SSD was recognized successfully and worked with full speed.
But with embedded SATA cable and power cable, it did not work properly.
However, when I pull out the two embedded cables from cable holder is attatched to HDD slot, all problems are cleared surprisingly!
I don't know the reason why it works.
Anyway, this trick is effective to me.
Thank you for the reply. I tried that but didn't work.
I also plugged in SSD to my desktop. It didn't work over there as well.
So somehow NUC killed my valuable SSD.
So I installed OpenELEC on USB drive and it works.
I also purchased a new 1TB HDD. See how that one goes for next couple of weeks.