Very confused about frontview backup

I  need to create a new backup set  from my RN104 before I upgrade my disks.   The problem is when trying to backup to a USB 3.0 drive  formatted ext3 I am getting about 10MB/s.    Network backup   is about 5MB/s,   rsync is about  10MB/s,  linux cp gives me about 15MB/s, however Windows file copy which should be really slow gives me 40MB/s.  Can someone   please tell me how this is possible and   tell me the fastest method to backup 7TB to a 8TB drive.  Thanks in advance.