there's no way, to connect a harddiskdrive to 500x units.
The reason is the LSI-chip. In 500x-units it's the 860x, in HD-recorders it's the 865x.
The features of the chips can be read here ---> http://www.lsilogic.com/products/recorder_dvd_dvr_dtr/index.html
The 865x has the capable of handling a harddrive ... the 860x can't.
There's no way, to upgrade a 500x-dvd-recorder to a hd-recorder ...