First off do you own the 2 receivers? If no then the suggestion you keep the minimum subscription package going until you can offload what you want to view again seems like a good answer.
Now if you own the receivers here is what I did. Dish required me to change over to the HD receivers. On 1 of the 4 512 receivers I had over 100 hours of TCM movies which I did not want to lose. Being there is not a way to transfer from my old receiver to the new one, I had to come up with a solution. I was lucky being that was the only receiver I owned out of the 4.
I disconnected the 512 completely,coax and power and set in a corner for a while. I eventually hooked up the receiver to a DVD recorder and applied power only to the receiver. The screen will eventually say it cannot find a signal. At that point I pushed the DVR button on the remote and there was all my recorded material. Inserted blank DVD-R and went about getting everything I wanted saved transferred to DVD. Yes it was a one program at a time process but I didn't just lose everything. Yes picture quality is not that of a factory DVD but it is watchable. Being everything I had on the receiver ranged from silent movies up to 50-60's scfi and horror B movies, I am happy.
Now as for an external hard drive, I am assuming here now, it would act like multiple hard drives in a PC. Does using an external have a separate listing under my recordings with the DVR button? If everything shows(internal and external drives) under the my recordings and you don't hook up to Dish I don't see why it would not work. Just that you will be spending a lot of time switching out blank DVDs and hitting buttons per item recorded. Personally I set timers and/or wrote down total time to record the movie/event and time when it would be done. Nothing automated.