i think the program providers will hamstring any universal standard.........
If you are speaking of encryption, that is entirely different than hardware standards and drivers for the operation of physical USB device.
As I think I saw Scott post somewhere. Nobody has hacked the encryption on any of Dish's DVRs to date. An external drive 'does' provide easier access to the physical media for those willing to try, but I don't think Dish (or any DirecTV ...any cable company with external storage for that matter) would be going forward with external storage if they weren't supremely confident in their encryption standard.
Time will tell. All I'm doing is providing my advice in purchasing hardware. Better to buy quality components and build your own, than to buy the "deal of the week" and save $20.