Why won't it let you sir? Do you have both USBs plugged in?
You may or may not need a transfer cable, not sure on that.
Worse case scenario is you could plug your usbs onto your computer and transfer them that way. I know you can transfer data from the PC to the console on a USB/transfer cable and I'm fairly certain you can shift it all to another USB if you have admin priviledges. You may need a program to view all this info though.