How to partition an existing drive

Hi guys,
Can you help me with my problem? Currently I have my 500 GB hard disk partitioned to Drive C where the OS resides and Drive D for file storage.
Is it possible to partition my Drive D again into Drive D and E with 50% capacity each. Will it affect my existing files or should I back-up it first to an external drive?
For your suggestion please.
Thanks,
Owen
