Cause:

• Message log may contain the following:

MSSQL: warning: explicitly included database 'XXX' on DEFAULT server instance appears to be offline, will try anyway
MSSQL: error: backup of database 'XXX' on DEFAULT server instance failed unexpectedly: An exception occurred while executing a Transact-SQL statement or batch.: because Database 'XXX' cannot be opened because it is offline.

• Databases and all of their dependencies must be online to be backed up through SQL Server.

The following excerpt is from the [Microsoft knowledge base article](http://technet.microsoft.com/en-us/library/ms175477$SQL.90$.aspx){:target='\_blank'} on the subject of "Backup Overview":

Offline Data Cannot Be Backed Up

Any backup operation that implicitly or explicitly references data that is offline fails. Some typical examples include the following:< > You request a full database backup, but one filegroup of the database is offline. Because all filegroups are implicitly included in a full database backup, this operation fails.
To back up this database, you can use a file backup and specify only the filegroups that are online.You request a partial backup, but a read/write filegroup is offline. Because all read/write filegroups are required for a partial backup, the operation fails.You request a file backup of specific files, but one of the files is not online. The operation fails. To back up the online files, you can omit the offline file from the file list and repeat the operation.

Resolution:

Exclude offline databases from backup through RVXConfig or bring selected databases AND ALL OF THEIR DEPENDENTS online before attempting backup