Advantages and Disadvantages
Historically, external backpacks were all that were available, and for that reason, many people mistake external frames as being "old technology." They're not. If most of your hike is on developed trails, and you will be hiking for long distances, then an external pack is the best choice.
External frame backpacks hold most of the load over your natural center of gravity, so you can walk naturally. With the frame on the outside, it creates ventilation coming off of your back, and allows the air to blow in between your back and the pack. This causes less sweatiness.
External packs typically have more pockets on the outside, so you can get at your gear easier. Another advantage of the external pack, is that the frame gives you extra places to tie on your gear like pads and jackets and fishing poles and sleeping bags and tents and whatever else you might want.
If you use your pack for climbing, ski touring, or other activities where your pack could get snagged on branches or rocks, an Internal Pack is the best choice. It rides close to your body so you have better balance on difficult terrain. It is also easier, because it has no exposed frame parts, to load into canoes, planes, busses, or cars. Internal framed backpacks really became popular beginning in the 1990's. There is still a "coolness" factor about them, because you usually tend to see them in rock climbing and adventure photography.