DVR recording amount & external drive

I searched the forums but did not find the answers I needed, so I am posting myself.

I have the Scientific Atlanta Explorer 8300HD DVR. I have seen references on line that the DVR will hold 20 hours of HD content. Is this the right amount?

I have also seen on line that a Western Digital My Book AV can be used as an external drive to increase the amount of stored HD content. Does one of these work with the Scientific Atlanta Explorer 8300HD DVR?