The problem with using an external drive with a non-dvr receiver is that the recordings are linked to the receiver rather than your account. Don't plan on being able to play archived content if you end up replacing your receiver. On the other hand, an external drive with a DVR receiver is linked to your account. I still can access programs on my external hard drive that were recorded with my vip-622, my Hopper with sling, and now my hopper3.
I have the Olympic opening ceremonies, rose bowl parades, etc. from the last 10 years. It is well worth the extra monthly cost and it keeps me tethered to dish.