Originally Posted by: Insane Geezer 
Sounds more like a corrupt file. You will have to go into Xbox 360 Settings > System > Storage... Somewhere in there you can look at the save files or game files and if there's a "!" on a file, it's corrupt. You might have to re-install the game to your drive, that is, if you installed the game to your drive.
If the game is not installed, it maybe that your disc has a scratch.
Search for a marked file with the " ! " was the first thing i've tried, because that have already happend to me before with other game, and the solution i know so far is deleting it :(
The thing is that there were NO marked file anywhere in the forza 4 folders, and the save file and the disc2 installation was in a flash drive that used to use before the HDD (may be the main cause)
After some time reading in the forums and commuities, the only solution was reinstall forza 4 completely and install everything in the HDD.
At least was able to keep some olders files like Tunes files, Custom stickers (group layers in the u.s version, i guess), Ironically the catalog of designs FACKYOU GATES, photos and Ghosts
But my 130+ hours disappeared for good.
I REALLY apreciate the support of you guys! thank you very much!