In medium saucepan, combine cornstarch, Old Bay seasoning and dry mustard over medium heat. Stir in milk until smooth. Add butter, stirring constantly. Bring to a boil over medium-high heat. Boil 1 minute. Remove from heat. Stir in 1-3/4 cups cheese until melted. Add cooked pasta. Gently fold in lump crab. Pour into greased 2-quart casserole. Sprinkle with remaining cheese. Bake uncovered 25 minutes, or until lightly browned.