Building fires or other preventable disasters
Unpredictable natural catastrophes
Duplicated system processes tolerate single system process failures, as well as single machine failures. Attaching the duplicated mirrored (paired) machines to different power supplies tolerates single power failures. By keeping the mirrored machines in separate buildings, a single-building fire can be tolerated. By keeping them in separate geographical locations, natural catastrophes like earthquakes can be tolerated.