WBADMIN don't do incremental backup

WBADMIN use Volume Snapshot Service on both side to mimic what we have the use to call incremental backup.

• On the source side (the disk your are backing up), wbadmin use VSS to backup blocks that have changed since the last backup.
• On the target side (where you are backing up), if VSS is enable you can use the previous versions to access old versions of your backup. Network share don't support VSS then you cannot access old versions of your backup, you must backup in different directories. 

Every 14 days or every 14 backups, I don't remember, wbadmin does a full backup. It read all blocks on the source and overwrite all blocks on the destination.

To use the VSS feature, you must use the -vssFull switch.

If you need to do an occasional backup and don't want to disturb your daily backup, you must use -vssCopy switch (or not use -vssFull for old wbadmin than don't know the -vssCopy switch) and the historic will not be updated.

Be careful, each version of wbadmin (on W2K8, Vista, Windows 7) are a bit different and provides different switches.

If you want to use wbadmin in a script to do complex backups and send email reports you should try  MKSBackup instead. MKSBackup is a front end for wbadmin that handle multiple target directory depending the day of the backup and generate a very detailed mail report with an accurate mail subject about he success or the failure.