If the media family spans multiple volumes, you need to keep track of each and feed them to the system at the appropriate time, in the right order. If you get it wrong, you'll get error messages such as the ones below:
The restore operation expected a different volume number than the one it was given. The expected volume is specified in the message. Replace the volume with the expected one and continue.
The backup set starts with an earlier volume than the one in the backup device. Replace the volume with the expected one and continue.
You've probably got the tape sets mixed up. The volume in the backup device is not a member of the media set being processed. Remove the volume and replace it with the expected one. If you're using disks, reissue the command, making sure you only designate those devices that are part of the same media set.
You probably forgot to change tapes before telling the backup operation to continue. In any case, the volume in the backup device can't be written to as part of the continuation set, because it is part of the same media set.
The key to avoiding these problems is to make sure you keep your media sets together and in the right order. If you're spending a lot of time on this, you should probably seriously consider a tape library.
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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years, he has been a freelance writer specializing in storage and other computer issues.