I can't believe no one has answered this easy question before now. The answer is yes you can repartition your drive with files on it. I have done it many times. But you must understand that every drive already has at least one partition to encompass the entire drive. When you repartition your drive you are taking and giving (or swapping) "free space" from one partition to another partition. Simply decide how much free space you need to run the OS comfortably and then put the rest of the free space as a new drive; remember to format your new drive before trying to use it. Peace and Love! Enjoy. 8)
