An enterprise storage system stores and gives access to valuable user data and hence is required to operate as a highly available system. One of the key steps to desired reliability is to perform continuous monitoring and diagnosing of a system state. A standard approach to technical diagnostic infers a diagnostic model. However, the complexity of a storage system and the lack of a common diagnostic methodology pose a challenge in creating such a model. This paper presents an ontology-based approach to diagnosing of a storage system, which integrates an expert knowledge of diagnostic parameters, typical storage configurations and common failure modes. An implementation of the approach is presented, as well as considerations of the benefits and limitations of the ontology-based diagnostic model.