The amount of data generated each year continues to increase. Some of this data must be preserved, and this becomes a significant issue when critical data are involved. Data storage systems must protect against risks that affect both data and infrastructure (primarily the storage media). To fulfil this requirement, the system should offer both data versioning (to restore previous safe versions) and replication (to mitigate device failures). These two functions should be dimensioned based on risk analysis, which may, however, result in complex software/hardware architectures. In this paper, we propose a simple graph model that accounts for risks to both data and devices and facilitates the design of a data storage system. It is complemented by a matrix model that allows analysis of specific properties. The proposed models enable the detection of suboptimal architectures and facilitate understanding of the operations of the backup system. In addition, this study can also support future work on risk management in backup systems.