Hello,
Are you backing up the clustered drive from the virtual server rather than backing up from individual nodes.
Please refer the below stated Technote-:
http://support.veritas.com/docs/246640
How to properly protect a Microsoft Cluster Server through Backup Exec 8.x , 9.x, and 10.0. for Windows Servers
NOTE : If we do not receive your reply within two business days, this post would be marked assumed answeredand would be moved toanswered questions pool.