By default Microsoft is limited USB data transfer size in windows 7 to 64 KB ,Now Microsoft has released a new update for Windows 7 to increase the performance of USB storage devices by increasing the maximum transfer size from 64 KB to 2 MB in the Usbstor.sys driver.

According to Microsoft, installing this hotfix will enable you to update the maximum transfer size of USB storage devices in Windows 7 or in Windows Server 2008 R2.
After you install this update, you need to manually increase the maximum transfer size from 64 kilobytes (KB) to 2 megabytes (MB) in the Usbstor.sys driver.(by editing registry values)
DOWNLOAD KB 2581464 hotfix