External Frame Backpacks differ from internal frame Backpacks in several ways. The bag on an External Frame Backpack is attached to the outside of the Backpack frame with a series of "pins." The Pack bag can be removed from an External Frame Backpack.
External Frame Backpacks allow you to attach a wide variety of gear to the outside of both the Bag and the Frame, including a tent and a sleeping bag. On some External Frame Backpacks, the bag can be removed and the pack frame can be used to haul bulky loads.
Choosing an Internal Frame Backpack, or an External Frame Backpack for use as a Bug Out Bag is a matter of personal preference as both will work extremely well.