Backing Up to an Existing Media Set
When you are backing up to an existing media set, you have the following two options:
Append to the existing backup set.
To make the best possible use of the available space, new backup sets typically are appended to existing media set. Appending to the backup preserves any prior backups. For more information, see Appending to Existing Backup Sets.
Appending, which is the default behavior of the BACKUP, can be explicitly specified by using the NOINIT option.
Overwrite all existing backup sets with the current backup, leaving the current media header in place.
SQL Server backup has safeguards to prevent you from accidentally overwriting media. However, backup can automatically overwrite backup sets that have reached a predefined expiration date.
For tape headers, leaving the header in place can make sense. For more information, see Overwriting Backup Sets.
Overwriting existing backup sets is specified by using the INIT option of the BACKUP statement.