I won't quote the whole e-mail, but he are the interesting parts of one I received from Indigial;
The AVHD drives are specially formatted to maintain the 5C content protection and are not user installable or replaceable.
The AVHD is designed specifically for today's new Digital Cable Ready HDTV's. Unlike the 3410, the AVHD incorporates 5C encryption and is able to record authorized copy protected programming from DCR TV's. Although the LG product has a built-in tuner, it was designed in 2003 and its tuner/decoder is already out of date. It also acts as the controlling agent for the incoming video, using the HDTV as nothing more than a monitor. I believe it even converts the signal to analog before output to the TV. If you have a "HD Ready" TV, this may work great. If you just spent the money on a new DCR HDTV with a top-of-the-line HD decoder and built-in TV Guide, then it is not even an option.
Notice the "specially formatted" part.
Also notice "not even a option".
Others have mentioned trying accessing the HDD with Lynux and trying a TiVo hack.