When using the SMS_SITE_BACKUP component to perform a Configuration Manager 2012 backup you may receive the following errors:
SMS SQL Backup service on the SQL Server \\<SQL Server Name> failed to backup the site database CM_<Site Code>;. The error reported by the service is Error: SQL Writer not found.. Backup operation is not completed.
Site Backup failed. The error reported by the service is Error: Sql Server could not prepare for the Backup.. Backup operation is not completed. Please see smsbkup.log for more information.
To resolve this simply configure the NTAUTHORITY\SYSTEM account to have SYSADMIN permissions on the System Center 2012 Configuration Manager SQL Instance. This is required for the VSS writer to backup the SQL database.