About possible data loss causes

If tape drive access protection is not enabled on the NetBackup media servers, the following may cause data loss: configuration errors, incorrect paths, multiple master servers, incorrect Shared Storage Option configurations, and third-party or operating system utilities.

If access protection is enabled on all NetBackup media servers, the following can cause data loss: any third-party or operating system utilities that run on the server that runs the NetBackup backup job.

Unfortunately, data loss cannot be prevented only recognized after the fact. NetBackup does not remove catalog information about the backup sessions that were lost. Use the bpexpdate command to expire the images for the lost backup sessions.