Don't add a second hard drive. Double the noise, heat, and drain on the power supply. A single 300gb drive should handle whatever you want to put on it. Do a google search for 'hinsdale howto' to get instructions on how to replace a DTivo drive.
265 hour (Samsung 300gb) Samsung S4040R zippered, network enabled,
220 hour (WD 250gb) RCA DVR40 zippered, network enabled