I had similar - but not exactly the same - issues with my DN2820FYKH when I tried installing Win7 64bit. Drives disappearing, Windows install freezing 1/2 way through, etc. I even purchased a new HDD & RAM from the compatibility list and had the same result.
The solution to my problems was a replacement NUC - the new unit installed Win7 fine using all the exact same components.
You may have a defective unit...