Incremental Move and stop

Hi guys. I've been trying to figure this one out for a few days. I've tried everything that was suggested on here and stackoverflow and quite a few other sites and nothing seems to work. I'm trying to make a cube that has different parts. That's the easy thing. The next part is the difficult part. One of these parts has to move when dragged by the player but needs to stop when the part is on the edge of the cube. I've got that to happen but it happens after the object is no longer in the vicinity of the cube.

This is the code I'm working with

 private Color highlight = new Color (0, 55.0F, 0);
         private float moveX;
         private float moveSpeed = 1.0f;
         public float minX, maxX;
     
         //Highlight the object we are going to move
         void OnMouseOver() {
             renderer.material.color += highlight;
         }
     
         //Remove highlight when mouse isn't on the object
         void OnMouseExit() {
             renderer.material.color = Color.white;
         }
     
         // Use this for initialization
         void Update() 
         {    
             float mouseX = Input.GetAxis("Mouse X");
             
             if (mouseX < 0)
                 moveX = -50.0f * moveSpeed * Time.deltaTime;
             else if (mouseX > 0)
                 moveX = 50.0f * moveSpeed * Time.deltaTime;
             else moveX = 0.0f;
         }
     
         //while dragging mouse move object
         void OnMouseDrag()
         {
             Vector3 move = new Vector3(0.0f + moveX, 0.0f, 0.0f);
             transform.Translate (move * moveSpeed * Time.deltaTime);
         }
     
     
         void OnCollisionExit()
         {
             transform.Translate (0, 0, 0);
     
         }

Thank you for any help you can provide regarding this matter.