The present invention is notably directed to a method for synchronizing proprietary data in an external cloud provided by a cloud service provider with data of a private storage system. The method comprises, at a synchronization system: copying outward data from a flow of outward data sent from a private cloud to the external cloud, the outward data being proprietary data of an entity that owns data of the private cloud. Next, and in parallel to copying outward data: the synchronization system compares copied outward data with data stored on the private storage system, to determine whether the compared outward data are already replicated in the private storage system. Finally, if it is determined that the compared outward data are not yet replicated in the private storage system, it instructs to store the compared outward data on the private storage system. The present invention is further directed to related systems and computer program products.