If you've never played the game on this console/hard drive then go to the system memory and delete the B3 saved game item. It should be seperate to the game file itself. If other people use the console they may have used up the slots.
Usually when you get "unusable" it is because you have played the game and then tried moving the hard drive to a different console or had your console repaired. B3 doesn't allow game saves to be moved around and played.
Did you have the disc version before? If someone used up the profile slots or someone else on another gamertag on your console maybe used the slots. Because it's an original game there are only 3 slots per console, not 3 slots per gamertag.
My guess is someone had B3 on disc and played it on the hard drive/console you use before you got it, but the console has been away for repair and now you have the 360 and hard drive and have downloaded it. Maybe you upgraded the HD from a 20G to an elite and copied the files over, that would make any old B3 saves unusable too.
I have a lot of B3 vids on Youtube btw, some that help with the harder parts of unlocking the Euro
http://www.youtube.com/profile?user=Fnordinary&view=playlists
