If the data is scattered all over the disk, that maybe the largest unused portion at the end of the disk. Try defragging with Defraggler, this should move most of your data into one continuous block at the start of the drive. Once this is done, you should be able to shrink the partition you have (assuming that right now the drive has one large partition that's the entire volume of the drive). Once shrunk, you should be able to make a new partition in the space you've freed up. Of course it goes without say that you should probably back up your data before messing with partitions. It doesn't happen often, but occasionally you hear of someone losing data doing things like this.