Hello,
What type of backup are you running? Is it an overwrite job? In case you have relabled the media, the new label is not actually written to the media until an overwrite operation is performed. For more information, refer to the following technote:
Title: How to label media or rename media labels
http://support.veritas.com/docs/273539
In case of further doubts on this issue, revert with details.
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Regards.