A method for writing data to a memory device comprising a first and a second memory device the first memory device comprises data blocks numbered with block numbers and the second memory device comprises at least one reference calculated from a data block digest and its physical block number. The invention comprises at least the steps: calculating the digest from at least part of the data block content, receiving at least one physical block number, to which the data block contents in the first memory device is stored, encrypting the data block content, storing the data block content to the first memory device to the position pointed by the physical block number, and storing or issuing a command to save the digest, or a number derived from it, and at least one said physical block number to the second memory device. A system, computer program, and server are also presented.