Mobile devices are increasingly relied on but are used in contexts that put them at risk of physical damage, loss or theft. We consider a fault-tolerance approach that exploits spontaneous interactions to implement a collaborative backup service. We define the constraints implied by the mobile environment, analyze how they translate into the storage layer of such a backup system and examine various design options. The paper concludes with a presentation of our prototype implementation of the storage layer, an evaluation of the impact of several compression methods, and directions for future work.