I have a Osprey Atmos 50 for an overnight pack and it has two options for a hydration bladder. One is a pocket inside the main compartment and the other is between the trampoline mesh and the main compartment. It looks nearly impossible to remove and replace the bladder in the pocket inside with a full pack. It is pretty cumbersome to R&R from the space behind the mesh, especially if the hose is threaded through the specified opening.

My question for those that use hydration bladders is, do you try to fill it with it in place or do you remove them? It seems there is the risk of spillage on to the items inside and I can imagine that with some water sources it simply wouldn't be possible.