My friend did this and has the hard drive in a seperate box outside the unit in a removable HDD bay, he can swapp HDDs out and archieve his shows that way or transfer them to DVD by placing them in a bay on his computer.
Imagine the money would make if Dish did their own removable HDD system... sure there would be the techno guys who would work around them, but the average user would buy drives directly from them. One thing I have learned with DVRs, there is NEVER enough space for everything.