Weaksector, if you only have NTFS partitions then it cannot be a FAT16/32 filesize problem
However, it can be a problem with Alcohol itself, that might not be capable of handling those big files. Are you using the latest version ? Has anybody succesfully made such a big image with Alcohol120% ? You don't need a game dvd to try, I've got some free dvd's of a PC mag and if I'm really bored I might try to make an Alcohol image of one of them