This adapter from Amazon shares the same chipset as the one Dish sells, it's the same price but if you have prime then you save on shipping.
Yes, as a matter of fact if you ever had a EHD hooked up to you 722k it should be able to read its contents since the formatting is the same.
If cheap as posible was your objective then I would have gone Wally, you hook up a EHD of your choosing and pay a one time $40 fee to activate DVR features, it's only $5 per month compared to $10 after that (I believe that if you are an old enough customer you might even get the first receiver free discount, making it $0).