Fun fact: Dish Network charges a 40.00 external hard drive activation fee, the ability to use an external is disabled until you pay it. DirecTV charges no such fee. Which is another reason why I prefer D* over E* even if I didn't work for D*.
Fun fact debunked: DISH only charges an EHD activation fee on their ViP211(k) non-DVR receiver that converts it to single-tuner DVR functionality. This is a capability not shared by the DIRECTV H2x series of receivers.
EHDs are free to activate on all DISH HD DVRs and doing so doesn't disable the internal drive nor force you to reprogram your recording preferences. Additionally, DISH EHD recordings can be shared among all HD DVRs on the account; something DIRECTV hasn't seen fit to implement.