I have the original 250 GB HD from an upgraded S3. Can I replace the stock 160GB TiVo HD drive with it? Can the 160 be used as a backup drive for the S3? Also, before I buy the TiVo HD, are there any problems of significance - I see there is an update related to the SA cable cards (I have SA) to fix some initial problems. Also, considering buying a 400GB TiVo HD unit from DVR Upgrade (price looks decent), any cautions? Thank you.