A number of issues could be the cause of slow copy speed. You will have to do some research via
www.google.com/linux, using search words 'sata copy speed'.
Some sata controllers are known to cause slow copy. If your controller isn't one of those, then you may be able to tweak hard drive performance via hdparm. It could be a matter of how many other processes are taking up cpu cycles. Or, you may be able to simply renice the copy process to give it higher priority.
