WD Green is a slow hard drive. it's designed for storage rather than booting or running games from. it might be struggling to load the textures fast enough as you progress in the game.
What's your definition of slow?
I have a WD Caviar Green and I am running GTA IV maxed with 54 FPS average on FX-8350 and HD 7950. --- Xbox Live GT:ScaryPillow , iHaruyuki TERA: LoL : ScaryPillow Maplestory: MLGLordly 16x I/L Arch Mage