If you create a partition manually, all the data on the drive will be lost; however, there are partition programs that will allow you to partition without interfering with files. I don't know of any offhand, but I'm sure others here can tell you.
Partition magic 8.0 or higher, about $50 or ranish.com. Free.
Be very careful and follow the instructions. BACK UP FIRST!! I have seen them fail and result in total data loss.