What two (or more) systems are involved in this process?
This happens to me almost every time I change the batteries. I usually force the little door open. Depending on what type of detector you have you can just grab the whole thing and twist it counter clock wise to remove it from it's base and off the wall. If you do this and it is attached to wires, the wires will plug into the back something like a cell phone charger. Don't be afraid to use some force to open the little door the ABS plastic is harder than you might think.
Stutt, your concern should be not the results of the fire, but the cause of the fire. If there is a true fire in the cargo hold, the chances are that everyone will be dead before the aircraft can make a safe landing. The aircraft is going to be at least 15+ minutes away from a suitable airport in an internal flight, taking into account descent time, and in a longer hail flight, very long. If there was a fire on board, it would have caused far too much damage to the planes control structures, cabin etc within these 15 minutes to really land safely. You have maybe a few minutes at most to land But anyway, how many major air accidents have there been down to fires in the cabin or cargo area. You seem to have a bit of an obsession with 'pax aircraft fires'. There are far more addressable issues which could easily save more lives.