I install the latest version of Ubuntu on a computer that previously had Windows 7 and Vista on it. Ubuntu is install in a separate HDD. My problem is that when I boot the computer the Windows boot manager appears with Windows 7 and Vista as an option to select, Ubuntu is not on the list. Thus I can not boot into Ubuntu. Any suggestion what to do next?
You need to install a boot manager, like Grub or something similiar.
Insert your Ubuntu CD, reboot your computer and set it to boot from CD in the BIOS and boot into a live session.
You can also use a LiveUSB if you have created one in the past.
Install and run Boot-Repair
After this, click "Recommended repair" and apply.
If you are willing to use the advanced options, make sure you leave the "Reinstall GRUB" checkbox ticked.
Now reboot your system.
The usual GRUB boot menu should appear. If it does not, hold Left Shift while booting. You will be able to choose between Ubuntu and Windows.