often in dasd subsystems , circumstances can occur which prevent the full transfer of the required data from channel to subsystem during a write operation . the disclosed methods prevents this data from later being read and treated as valid data by the host processor . this is achieved by marking data as invalid within the storage subsystem once it is determined that a channel error has occured . subsequently , upon reading that data , the host processor can be made aware of the data invalidity and treat the data accordingly . in a second embodiment , invalid data is discarded rather than stored over the previous valid version of the data , before it is ever stored on dasd .