I'm looking to purchase a used TIVO Series 3 or HD unit on Ebay. As the hard drive in the unit may need to be replaced not long after I purchase it, I was wondering if these units can operate using ONLY an external hard drive such as the WD My DVR Expander? Instead of replacing the dead internal hard drive can I use only the external hard drive? Or will the external hard drive only work when paired with an active internal hard drive? Thanks.