I'm agent Nahman Jayden, eff-bee-eye.
All of your custom maps, games, films, and screenshots are on your harddrive, under halo 3. When ODST comes out, the second disc (matchmaking,custom games, etc) is halo 3, hence will be halo 3 in your harddrive while the campaign disc (first disc) wil be in halo 3: ODST in your hardrive