NUC6CAYS does not support RAID. I looked in Visual BIOS and saw only AHCI mode for the SATA controller.
To run Windows 10 properly the disk capacity needs to be 64GB or more. 32GB is too little as you noted. I recommend getting a 120GB or 250GB SATA 2.5" SSD to use with the NUC6CAYS and switch off the internal 32GB MMC flash as it is quite slow anyway, and good probably for Linux (e.g. Ubuntu) but not so much for Windows 10.