When you code DISP=MOD and the volume information is for a multivolume data set, normally the first volume(s) will be mounted on the devices(s) allocated. Then, if the data set is opened for output, OPEN starts with the last volume. If the number of tape volumes is more than the number of allocated devices, the system asks the operator to demount the first volume(s) and mount the last. To have the last tape volume mounted without first mounting and then demounting the earlier volume(s), code VOLUME=REF or DEFER in the UNIT parameter, or a volume sequence number in the VOLUME parameter.