The invention relates to taking a back-up of original data and deciding whether the original data has been altered. A back-up of the original data is taken by copying the original data from one storing unit to other storing unit as back-up data. Once the copying is over, it is decided whether the original data has been altered based on a comparison between the original data and the back-up data. All the requests for access to the original data received while the original data is being copied are denied but request received while alteration of the original data is being decided are allowed.