I created two partitions of 500GB HDD and installed Ubuntu 12.04 on one of it.....
Where can I find the other partition that is not used?
Install the gparted
package. One way to do this is in the Terminal:
sudo apt-get install gparted
Then run GParted, and you will see your partitions.
Open Disk Utility. There you can view, delete, mount, and format partitions. You can also get information about your hard drives.
If you need to see partitions, I think you should run the palimpsest
tool. It should be somewhere in system settings, and you need the gnome-disk-utility
package (should be installed by default).
Then you will see your partition, format them, etc. Be careful before touching disks with that of course.