I am copying 2 virtual machines from one SSD to another SSD using Windows 7 64bit. The data is about 40+GB. My virtual disks are not split; one is 30GB and the other 8GB. Windows started copying at 200MB/sec and 30 mins later I am at 20MB/second, and still not done. Copying the same data over my 1Gbps network was less than 15mins (SSD to SSD). The operating system is not on these disks, and I am not using them while I am doing the copy.
Try connecting the drives to physically separate SATA controllers. If this resolves the problem, then the cause is most likely that they were sharing the same controller and you were experiencing a bottleneck.
Also, any disk caching providing by your Operating System may be skewing the initial transfer speed statistics.