Thanks for the great tips guys!
I was able to re-purpose a 500gb Comstar external HHD that has a USB 2.0 interface to a PVR expander.
I took apart the original HDD which has a Hitachi HDD inside, I then went the local computer store and bought a Vantec NST300 which has USB 2.0 and e-SATA for $25.
The first time I hooked up the external HDD, it didn't work and the PVR seemed to have erased all the previously recorded contents. *panic*
So I unhooked everything and followed the instructions very closely and waited the proper amount of time for each step then viola!
The PVR was about 86% full with about 16 hours of HD recording on it. Now's down to 15% and I figure I should be able to have about 100 hours of HD contents.
One minor side effect of the upgrade was the "info" for the existing shows were erased, so it only shows the titles but no summary/plots anymore. The new recordings are all fine.
Thought I'd share my experience.
Sharp D64u 42" & 46" , PS3 , Toshiba Qosmio G40, Yamaha 6130B, Energy Take Classic