About protecting against storage device failure
Whether they are tapes or disks, when storage devices fail they are considered to be single points of failure. To protect against storage device failures, you should have multiple devices as backup targets.
A media server with access to only one tape drive cannot complete backups to tape if that tape drive goes down. To protect NetBackup against such failures, configure the media servers to access at least two tape drives. Use SAN-attached tape drives, which can be shared between media servers. This sharing ensures that the tape drives are accessible without needing large numbers of redundant devices. Typically, one or two redundant drives provide for resilience and allow restore operations to occur while backups are in progress. For example, if you configure four media servers to share five tape drives, backups can still happen even if one drive goes down. The backup may take longer, but it completes and your data remains safe. If media servers run backups at different times, the ratio of tape drives to servers may be even lower without risking backup failure.
AdvancedDisk disk pools can be created on individual media servers to protect against the failure of a single disk device.