Backup Exec gathers disk usage information for all attached storage arrays. Through statistical analysis, Backup Exec then estimates how much time remains before the disk space on all storage arrays is depleted. You can also specify how many days before low disk space occurs that you want Backup Exec to send an alert.
To configure an alert for low disk space on storage arrays
In the task pane, under Settings, click Storage Provisioning Alert.
Change the defaults as appropriate.
About predicting disk usage in the Storage Provisioning Option