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.