Only 3 partitions can be created on a Hard Disk using Windows 7,
4 for XP.
You probably had hit the maximum. If you don't see 3 Partitions
in your drive, probably they are hidden.
Actually, a maximum of four physical partitions may exist on a
disk, be it formatted in 7, Linux, OSX or on XP. The fourth
user-inaccessible partition on Windows 7 is used to store boot
data.