Slowd B2D Backup

I have an issue with backup speed.

When i try to backup Folder that located on Local Disk D:, to Backup to disk folder on the same disk, i get speed near 100MB/s

But, if i copy the same data from the same location to folder on D: i get 200-250MB/s.

Disk D: connected to media server via SAN.

Where may be a problem? I look at Perfomance Monitor and don`t see any load on system, no load on CPU, no queue for disk operations.