Instructions
Power on your XBox with the hard drive containing your player data attached and the USB flash drive inserted into a USB port on the console. Wait for the Dashboard to load. Move the analog stick to the right and select the "System" tab, then "Memory" by pressing "A." A list of all player profiles and game save data on the hard drive will appear.
Press "A" to select the hard drive data items from the list you want to copy. Select "Copy to USB Device," then press "A" and the console will begin copying the selected items to the USB memory device from your Xbox 360 hard drive.
Turn off the console and detach the Xbox 360 hard drive. Attach the second Xbox 360 hard drive to the side of the console and power on the machine with the USB flash drive still connected. Wait for the Dashboard to load, then select "System" and "Memory" with the controller. Select the USB flash drive by pressing "A" and select the data files you copied to the USB device. Choose "Copy to Xbox 360 Hard Drive" and press "A." The console will copy all the player data and game saves from the USB flash drive to the second Xbox 360 hard drive. You can use the data from the hard drive once the file copy process completes.