How to assign a drive letter to a Windows Portable Device?
I have found this from the Microsoft community website
http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/how-to-assign-a-drive-letter-to-a-portable-device/829e43b6-7210-4566-8e47-1ac147946f7b?page=2
Try using http://www.mtpdrive.com... It should do the trick....