The big stone only turns 100 rpm or so, the worst it will do is fall into the bucket...
Replacements are in the $45 area....
Best guess for 4 equal spaced cracks? Fell over on it's side, or as Phil says, some klutz cooked it welding the shaft.....
(Soggy water filled stone doesn't appreciate sudden heat, something about 30 cubic feet of steam trapped in 1 cubic inch of rock; just doesn't end well....)
PS: One bad feature about this design in general, the fast stone is at right angles to the big stone, if it is even slightly out of balance, it vibrates the big stone sideways making it impossible to put a good edge on anything...