Example, 6 x 600GB HDD in Single RAID-5 Array/VD ~ 3TB
By default, Windows Setup will use Basic Disk, only can use 2TB, and 1TB wasted.

1) Go to BIOS, change boot type from BIOS mode to UEFI mode
2) Insert the Windows Server 2012 media
3) In the Windows Setup screen, select to REPAIR your system and launch a command line.
4) Run diskpart in command line, run following cmd:
list disk
select disk 0
clean
convert gpt
5) Exit Windows Setup and reboot server.
6) Boot to OS Media and rerun Windows Setup again.
7) Setup Windows