If you mean you have a TiVo HD (and not an HD XL, which looks exactly the same except for the letters XL added on the front right after the HD), then you have a TCD652160. If your unit does say HD XL, then you have a TCD658000. But if the front has a clock display, and buttons around a circle, then you have a TCD6438250B. Those are the three Series 3 models. But if you meant that your original hard drive finally died, then there are many more possibilities and the model number is needed.