BACKUP always makes a full backup of the catalog store on the first archive device, and then backs up the data from the IQ store in parallel across all of the devices you specify. Blocks are not distributed evenly across archive media. You may have more on one device than others, depending on the processing speed of individual threads.
The distribution of backup data is important because
sets of files must be restored in the order in which they were backed
up. See “Restoring in the correct order” for
more information.
