Suggestions from newsgroups:
- check SCSI cables (disconnect the cables and connect them again);
- check if external devices are powered on;
- verify that the SCSI bus is terminated properly
If Backup Exec for Windows NT/2000 jobs are reporting hardware errors such I/O Errors, Semaphore Timeout errors, and Event ID 9, 11 and 15 the in System Event Log, this may be due the fact that the SCSI driver on the system is out of date. See Veritas TechNote ID: 231918 link.
As per Microsoft: "Event ID 15 indicates that the device is not ready. This can be the result of SCSI host adapter configuration issues or other problems. Check with the manufacturer for updated firmware, drivers, or known issues. This could also indicate a malfunctioning device."