Copy-only backups are new to SQL Server 2005. They allow a backup of any type to be
taken without affecting any other backups. Normally, a database backup is recorded in the
database itself and is identified as part of a chain that can be used for restore. For
example, if a full database backup is taken, any subsequent differential database backups
use this full database backup as their base. A restore process utilizing the differential database
backups would have a reference to the full database backup, and that backup would
have to be available.
Copy-only backups do not affect the restore chain. They are useful in situations in which
you simply want to get a copy of the database for testing purposes or things of this
nature. Copy-only backups are not supported via SQL Server Management Studio (SSMS)
and must be performed via the Transact-SQL (T-SQL) BACKUP command.
you can only take a full or a log backup with copy_only. Differential backup will ignore the copy_only.