n00bs on Ubuntu published a step by step guide about resizing your primary partition and add a second partition on your primary disk on Ubuntu

Here is a guide to resize your primary hard disk containing your Ubuntu installation and how to add a second partition (ext4) to the primary hard disk. To do this you need to unmount the hard disk first. The only problem is that a hard disk in use cannot be unmounted...

  HowTo Resize Your Primary Partition And Add A Second Partition On Your Primary Disk On Ubuntu