Why is my external hard drive not recognized by Windows?

I have a WD 500 GB external hard drive with all my files on it from my old computer. When I hook up the hard drive to my new computer (a Dell XPS 15 with Windows 7 Ultimate), I hear the noise that indicates a device has been attached and the light comes on on the hard drive and I can feel the disks spinning. However, the hard drive does not show up on My Computer and I cannot access the drive or anything on the drive.