i assume your talking about the .VSP files that netbackup creates when starts a backup either that or your logging is set to night.
for .VSP you need to have the master not use Open File backups or accomodate the space for these files. Alternatively you can install NBU on a different drive with more space.
If its the log files creating the issue you need to turn the verbosity down to 0 its likley set very high