My husband suggests Copy/Paste commands instead of Move because if you Move the files over then it is gone from the original location and the game might not work, also you won't have the original files as backup. You could try Move and see if the game works but he says most likely it won't. If the game runs off the USB after Copy/Pasting everything than feel free to delete the files on your C-drive.
If the game doesn't work, then you'll have to delete everything from the USB (here is where having the backup will save headaches!) and install the game CDs, in order, to the USB. My husband says this may or may not work while the game in installed on your computer. If it does, you just need to change the location to which the game installs on that page of the installer. If it doesn't work you could try the EA Download Manager or uninstall the game from the computer (backup your saved game, downloads, exports, screenshots, vidoes folders) and then install it to the USB.
Let me know if this helps.