The usb 3.0 high bandwidth allows for really fast emptying of the fast disk cache in the initial moment (burst rate), and leave the mechanically slow hard disk platter to transfer small files, requiring constant stop to seek new file. In contrasts, in usb 2.0, the mechanical hard disk also transfer at 15MBps but since bandwidth bottleneck restrict the emptying of the cache in burst at the beginning, the data in the fast cache linger and is transfered at the same time as the platter data to saturate the 30MBps, giving it a more average speed. Now, if you have larger continuous file, like a any file over say 0.3GB. Because there is little seek for small file, hard disk platter will rotate without stopping. usb 3.0 will not be a bottleneck because the interface bandwidth is greater than the hard disk output. On the other hand, usb 2.0 is bottlenecking the mechanical hard disk because of its limited bandwidth. In large file, usb 3.0 will be about 180 to 300% faster than usb 2.0. Find a continuous large file to transfer. I will be surprise if usb 3.0 is slower than usb 2.0.