The tech support guy from my cable company said that the model DVR I have has a port for a SATA hard drive. Their was supposedly a new firmware upgrade that was released (or should be released) that allows you to plug in an external drive for increased capacity. He said that they don't support that feature through tech support, so they couldn't help you get it going.
Perhaps some internet research is in order... I have a motorolla DCT3416 HD DVR box. If I can just add an external drive that would be great.