If you want to do a full backups or log backups without disrupting your regular backup chain, SQL Server 2005 provides a good option to do it. Here is some words from SQL Books Online:
A copy-only backup is a SQL Server backup that is independent of the sequence of conventional SQL Server backups. Usually, taking a backup changes the database and affects how later backups are restored. However, occasionally, it is useful to take a backup for a special purpose without affecting the overall backup and restore procedures for the database. For this purpose, copy-only backups were introduced SQL Server 2005…
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The syntax is quite simple:
_BACKUP DATABASE database_name TO… WITH COPYONLY …
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Enjoy!