Western Digital My DVR / Tivo Expander External 500GB Drive eSATA or USB for $79.99 shipped.  Note that this Western Digital DVR Expander is available in USB version for DISH Network ViP Series HD DVR as well as eSATA version for TiVo 3.  They may also work with other DVR devices as well.  More information is available here.  Here is a screenshot for your reference.  The eSATA for TiVo 3 is also available here for $89.99 shipped.  TiVo itself sells it for a crazy amount of $199.99.

Originally posted earlier, it has been slightly revised and this is just a friendly reminder.