I can discuss this area a little as part of the work I do involves security DVRs and along the way I've bumped into discussions why this is so.
As you are on a CBS Forum, you can imagine the forum policy does not allow us to discuss getting around DRM or such.
In short, HD content is protected in many ways. We can state that but as you suspect, we can't discuss getting around that.
On checking manual for DMR-PWT520 DVD player it appears that you can only attach an external HDrive if you let Panasonic format same - then you can only use the hard drive on their equipment whjich seems a bit draconic! Can you get around this as I use my external HD for computer files and all sorts! Haven't been able to find out what externals they accept either! Cheers