Coat grill rack with nonstick cooking spray. Preheat grill to medium direct heat. Meanwhile, in large bowl, combine all ingredients except butter in order listed, toss to coat. Lightly coat a 12 x 24 inch piece of heavy-duty foil with nonstick cooking spray.
Place potato mixture in center of one end of foil, dot with butter. Fold foil over to enclose poatoes in a 12 x 12 inch packet, crimping edges to seal, leaving room for expansion.
Place packet on grill. Grill, covered, 35 to 40 minutes or until potatoes are tender, carefully turning packet over once. Season with salt and pepper to taste before serving.