Hi,

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Just getting started with SQL Server and trying to backup our database we're getting the above error message. It goes on to say "BACKUP WITH FORMAT can be used to form a new media set."
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Not much of an expert on this but I think you may just need to (a) Select the 'Overwrite existing media' button and (b) Select the 'Options' tab and check the 'Initialize and label media' box.

HTH,
Viv