1. Click on the Start Button
2. Type diskmgmt.msc in the Start Search field.
3. Press SHIFT+CTRL+ENTER to open Disk Management snap-in in an elevated state.
4. Right-click on the WD drive
5. Enter the amount of drive space to shrink in Enter the amount of space to shrink in MB:. Remember that 1024MB = 1GB
6. Click Shrink.
NOTE: When the Shrink operation is completed then you there should be unallocated space on the hard drive, refer to image below.
7. Right-click the Unallocated Disk Space to select New Simple Volume….
8. Click Next when the New Simple Volume Wizard opens.
9. Enter the disk size of the new simple volume and click Next.
10. Choose an available drive letter and click Next.
11. Perform a quick format the new simple volume as NTFS. Name the volume and click Next.
12. Review disk settings and click Finish.
13. Congratulations, you now have successfully create a new partition using the Disk Management snap-in with Windows Vista.
Let me know if you have any issues with this mate