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Surfaces and interfaces can dominate charge carrier transport dynamics in electronic devices, impeding realization of a material's full potential. Here, we investigate transport in a two-terminal diamond device comprising a conductive channel defined by a hydrogen-terminated diamond surface, bridging two TiC contacts. The surface charge distribution was imaged by monitoring the photoluminescence of nitrogen vacancy centers incorporated below the active device layer. A strong charge accumulation near the TiC contact/H-terminated channel interface is observed and is discussed in terms of deviation from Ohmic behavior evident in the DC electrical measurements. Small voltage steps applied to the device result in current transients due to carrier trapping at the contact/diamond interface. This gives rise to dynamic negative capacitance at low AC frequencies and is discussed in detail.