If i were you, i would do a clean install and then select the wd drive as system partition.
There are tools to mirror and move the os from one hd to another, which is pretty easy, im not a fan of that though.
CORSAIR 900D
ASUS MAXIMUS VI FORMULA
INTEL I7 4770K @ 4.5 GHZ
CORSAIR DOMINATOR PLATINUM 16GB @ 2400MHZ
SLI EVGA GTX 780 SC ACX
SSD SYSTEM: CORSAIR FORCE GT 120GB
SSD APPS: CORSAIR FORCE GT 240 GB
HDD STORAGE: WD BLACK 2TB
CORSAIR AX860I PSU
CORSAIR H100I 4 x SP120 HP