my brothers games and other belongings were stolen when he came back from service in japan. disc 2 was installed on the xbox and we only have forza 4s original disc since it was still in the system. the disc 2 content were on a drive which crashed. now
I cant use those cars anymore.
Go to settings/system/storage/hard drive, press y and clear your cache, try again. might or might not work, but it's definitely worth a try. You may have to redownload so of the latest updates, but no big deal. You won't lose any save files.
Disk two contains additional cars and tracks so you will be missing out unless you install it. On the main menu on the far right side is the option to install the 2nd disk if you have not already done so.
If you have not installed it (double check in game save info) and the option is not showing the go to forzamotorsport.net and post on turn10's forums and someone there will be able to offer more advice.