You do not say what form the magnesium is being stored in.

If it is powder, granules or other small particle size you are storing a bomb. The dust is explosive and very easy to start on fire.

If it is in the form of ingots, or blooms and you keep the floor clean then it is hard to start a magnesium block on fire unless you get the fire started from some other source. (Like trying to start a log on fire with a match)

You probably already know to keep water out of the building, both from piping or a leaky roof. Salt, flux or sand which can smother the fire is your best bet, but you must have a good sensing system and most importantly a well trained response team who can react quickly. A belt slinger on a truck or other mobile equipment might be better than trying to use a pneumatic conveyor type of sprinkler. A lot will depend on the shape and height of the building. ■