Find your Forza 4 car packs and select "Download Again." Choose your USB drive as the destination to keep these files separate from potentially corrupted system cache on the main HDD.
If you previously purchased DLC but it is not appearing, you can force a re-download directly to your USB drive through your account history. forza motorsport 4 dlc download fix usb
For those using the Xenia Emulator , the "fix" involves extracting the Disc 2 ISO and using the "Install Content" option to point the emulator toward the extracted car pack files. SUCCESS: Notes on how I moved Forza 4 to a new system. Find your Forza 4 car packs and select "Download Again