Low speed backup process in backup exec 2012.

Hello!

There is server Dell PE 720xd with array 8X2Tb SAS RAID6. Backup exec installed of this server. Array include big file. When I do simple copying inside array copy speed equally 200 Mb/sec. When I do backup Execbackup copy speed equally 10 Mb/sec (600-700 Mb/min), 10 times smaller than expected. Compressing is off. CPU-utilization - 5%. RAM-utilization - 1.5Gb, 6,5GB free. What is problem?