how do i install ubuntu 12.04.1 lts on separate hard disk?

mini tutorial

1. Get the iso from here --> http://cdimage.ubuntu.com/releases/12.04/release/

2. Get the unetbootin program from here --> http://unetbootin.sourceforge.net/

3. Get a Usb flash drive at least 2GB

4. Cook them together and make a bootable Ubuntu Live Usb (read here : http://unetbootin.sourceforge.net/#install)

5. Boot from bootable Usb.

6. Install Ubuntu on your disk.

As a safety key , before you boot from usb , you can unplug the disk that you don't want to install Ubuntu.

When installation finish , you must boot (first boot) from the disk you installed Ubuntu.
Then and as you already plugged again the other disk , open a terminal (ctrl +alt+t) and paste this command inside
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sudo update-grub
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then you can leave the disk where Ubuntu installed as the first boot disk , all other Operating Systems should be recognized.

Thanks.

Just select the other drive in the installer...

If you unpartition the drive in another OS, the installer should see the free space