A method and a storage server for backing up data, involves mass storage devices of users of the service that connect to the storage server of the service provider over a public data network. The customers store the data with their terminals. The data is stored to the file system of the mass storage device which encrypts the stored data and transfers the data in encrypted form over the public data network to the storage server. The storage server calculates error correction data from the encrypted data. The error correction data is stored in the mass memory of the storage server. When recovering the stored data of the users, the storage server requests the stored data of all the users whose data was used for the error correction data calculation over the public data network to be used for the recovery calculation.