Modes are widely used to structure the behaviour of control systems. However, derivation and verification of a mode logic for complex systems is challenging due to a large number of modes and intricate mode transitions. In this paper, we propose an approach to deriving, formalising and verifying consistency of a mode logic for fault-tolerant control systems. We propose to use failure modes and effects analysis (FMEA) to systematically derive the fault tolerance part of the mode logic. We formalise the mode logic and define mode consistency properties for layered systems with reconfigurable components. We use our formalisation to develop and verify a mode-rich system by refinement in Event-B.