My TiVo HD will use the internal drive and external together, so this would add 60 hours or so to the existing 20hrs of HD capability I currently have. The TiVo spreads the shows across both drives, so if you want to remove the external one you had better be prepared to lose everything you have recorded! Otherwise you have to watch everything first, then remove the external drive and you are back to the single internal one. Plus, the machine must be turned off to connect or disconnect it!
I think I will wait for a bigger one at a lower price!