Shovel inside or outside a pack doesn't really matter. Personal choice. Mine is inside because I normally use a shovel on the sled. Read below -
The comments below are based on owning, wearing an Avy pack. (ABS Escape 15) - they do not apply if you are not carrying an avy pack.
OK, here is something to think about. If your shovel is in/on you pack and you get stuck on the side of a hill, one would have to remove the pack in order to access the shovel. At that point, while digging, you would not have any of the protection the pack offers (remember - the pack is on the ground - my money says i/you won't put it back on, while digging). A slab breaks loose above you, you and the sled are tumbling helplessly down the hill and apart from your pack. At this point, the back is worthless.
The best option I've seen (IMO) and I follow, is I carry two shovels [and probes]. One of each on the sled and the others in my pack. When digging or searching, use tools from the sled - then my pack stays on my back and can help be if the side of the hill goes down. Further - if I ever become separated from my sled, I still have tools for survival, on my back with other survival equip (fire starters, space blanket, compass, etc.).
If I were to carry only one shovel and probe, it would be on my sled. Strapped to tunnel, in tunnel bag, under hood, etc.
Great question, good to see you are thinking about the right equipment, it is a very "sinking" feeling to find out that only 3 out of 10 have probes - after you already found out the guy that is buried left his probe at home. Thank God we found him, all is good AND all 10 are much better equipped when we ride in the Mtns.