Just for the sake of helping people out. I've already done this. I only wanted to use my HDD to transfer files from a dying 622 to a new 622 so I didn't lose anything. Hooked up my 500GB Mybook, reformatted to Dish and transfered files. Swapped the 622s (which took over an hour becasue the new one had L0.62 software and had to be updated several times before it would actually work...). Once it was all done, I plugged the MyBook into the new 622 and it told me "you have plugged in a EHD that is set to operate with a different receiver, do you want to update it to work with this receiver?" Yes. No reformat, just worked fine. Transfered all my vids back to the new 622 (took less time back than writting to it (which btw makes sense becasue apparently these drives read faster than they write for some reason, but that's another topic).
Anyway, when it was done I plugged back into my PC and it recognized it, but did not mount it as a new drive. I had to go to Drive manager to see it and reformat it. Side note, Dish system creates 2 partitions on the drive. The Primary partition is a 1MB partition, the secondary is the remainder of the drive. I'm no expert, but I would bet that the receiver pairing info is held in that 1MB partition.... Anyway, reformatted the entire drive, moved my files back to it and it's as if I had never hooked it up to the Dish. Exactly what I wanted!!! Later I may get a second EHD to use with the Dish, but I don't need it for anything.
Another side note, I had speculated that the 3 receiver swap could be overcome by reformatting the drive, that seems to be the case. While I was on the phone with the Service Tech, she told me that I could do the swap I wanted but (direct quote) "You can only swap the EHD 3 times before it will need to be reformatted." There you go... Anyway, hope this helps someone.