If they implement external HDD - they need to add a command/menu option for removing the HDD to ensure everything's done safely - just like in Windows where you have to click the little green arrow icon in your notification area to "safely remove hardware.
So, when you plug the HDD in, it's autodetected. When it's time to remove it, you need to access a menu item and wait for a prompt to tell you it's safe to unplug it.
If you unplug it without using the menu item, the box pops up a nastygram and tells you it was "unsafely removed," again, just like windows. (Who ever thought someone would use windows as an example for how something SHOULD work?).
Anyway, I don't think Dish would be dumb enough to have the 622 write "critical" files to an external device - tose would stay on the onboard HDD.