Are you using any of them in Raid 1 configuration? If so, what Intel Rapid Storage driver version did you use? I don't see a driver available for the NUC6i7KYK for use with Windows 2012 Server.
Well, right now I am using Windows® 10 and I haven't set RAID; however this version should work for you Download Intel® Rapid Storage Technology (Intel® RST) RAID Driver and I would recommend you checking this link RAID Setup Procedure for Intel® NUC Kit NUC6i7KYK for more information on setting RAID in your NUC. Stick to the instructions on this link and you should be able to set the RAID configuration.
Let me know how it goes.