Speed comparison of USB 2.0 vs USB 3.0 portable hard drive

Here are the CrystalDiskMark numbers of a USB 2.0 portable drive (Samsung G2 Portable 640GB):

and a USB 3.0 portable drive (Seagate Expansion 750GB):

So although the theoretical bandwidth of the USB 3.0 interface is much higher compared to USB 2.0 (48MB/s vs 480MB/s), it is limited by the throughput of the mechanical hard drive. So at best you are looking at a 2x throughput improvement when moving to USB 3.0 for mechanical disks.