Following Microsoft Knowledge base article help you resolving Event ID 11
http://support.microsoft.com/default.aspx?scid=kb;en-us;154690
The Event ID 15 indicates that the device is not ready. This can be the result of SCSI host adapter configuration issues or other problems. Perform the following steps to resolve the issue:
1.) Reduce the total number of devices on the controller.
2.) Update the SCSI Controller Driver and firmware
3.) Update the Tape Drive firmware
4.) Separate the tape device from a controller which also has hard drives connected to it.
5.) If possible, connect the library to another SCSI controller
Once above mentioned errors are resolved, run the same backup job again and verify the throughput.