A method for preparing data units for access in a data storage system is disclosed. The data storage system includes multiple storage devices having data units. All the storage devices of the data storage system are not powered on at the same time. The method includes preparing and storing the auxiliary data. The auxiliary data is prepared for a data unit on a storage device that will be powered off during an access request of the data unit. The auxiliary data is stored on the storage devices so that the auxiliary data is likely to be available on a powered-on storage device when the data unit is the subject of an access request.