1. First, you will need to download the current version of Ubuntu at Download Ubuntu Desktop. Select your architecture (32-bit vs 64-bit), and click the big orange button. If your computer has at least 2GB of RAM, select the 64-bit version of Ubuntu. If you are unsure of the flavor you need, select 32-bit as it will run even if you have a 64-bit processor. You may also download any distro release, such as Kubuntu or Xubuntu or Other Flavours.

2. Next, you will be downloading a large .iso file, about 800MB-1.6GB. It can take up to a few hours, depending on your connection speed. Let the file download. Then, you will be burning it onto a DVD or LiveUSB. The Ubuntu 14.04 .iso file is too large to fit on a CD, so you will need to either burn it to a DVD or else make an Ubuntu live USB using a flash drive that has at least 2GB. (links are for Windows, if you are switching from another operating system check the "Easy ways to switch to Ubuntu" section on the download page.)

3. If you are using Windows 7 or higher, you may double-click this file (assuming you don't have another program associated with it). Otherwise, install ImgBurn (sadly, not open source). You may also may be able to right-click and select 'Burn to Disk'.:

   
   ^{More instructions can be found here or here if you need them}

4. You now should have burned your disk. If you open the disk using your file manager, there should be a bunch of files like wubi.exe, autorun.inf, pics, and pool, among others. If you have just one file on the disk with the same name as your ISO, make sure you are using a disk burning utility. Do not just drag and drop the ISO file. It will not work.

5. Now, put the disc back in your optical drive, and reboot your computer. On a PC, you should have a key like Delete, F2, F10 or another key that will get you to your boot sequence menu. You can enter your BIOS menu by pressing one of these keys as soon as the motherboard splash screen appears when the computer is booting. Otherwise consult the manual for your computer or BIOS and look for something similar to boot from external media or choose a startup disk (⌥ or Alt on Macs) to find the key that is needed. Follow the screen prompts to boot from the optical drive your new disc is in.

6. Assuming you are not booting UEFI, press Shift once you get a purple screen with a picture at the bottom. Select your language with the arrow keys and Enter. Now, you should be confronted with a menu. You may try Ubuntu without installing, or you may install it. If you want to try it, choose "Try Ubuntu without installing", try it, double-click the Install Ubuntu icon on the desktop, and continue with this guide. Note that this test version is slower because it is running from a CD/DVD/USB, not your hard disk. Some versions of Ubuntu will give a graphical menu for this step.

   

7. Now, you will be confronted with a checklist. You may safely disregard the second option 'Install this third-party software', though Adobe Flash Player and some proprietary video codecs may not work, unless you enter the commands at the end of this guide.

   

8. Now it is time to partition your hard disk. If you want to avoid losing data or dual boot, this step is crucial. If you want to just clear out whatever is already on your hard disk, and use all of the space for Ubuntu, select that option, and skip to step 10. If you want to dual boot and trust the Ubuntu installer (not recommended), select the option to install side-by-side, and skip to step 10. If you want to manually set up dual boot, select the choice for advanced partitioning and proceed to the next step.

   

9. Find your current Windows partition. It should be the first or second under sda, and its type should be NTFS. Click it once, and click Edit to invoke the partition settings dialog. Reduce the size by at least 40,000 MB, or more if you have extra free space on your old installation. If you have a separate Documents partition, you may resize that instead. Then click in the free space you have created and click Add, setting the type to Swap area, and giving about 1.5-2 times your RAM (Note that the input box takes sizes in MB, not GB.) You will now have swap space. Click in the remaining free space and add an ext4 partition with the default size (maximum possible recommended), and the mount point as /. You may also want to do a /home partition, which is where all your user files in Ubuntu will go. If you make a mistake anytime during this, click Revert.

   For more detailed instructions (and with screenshots) on this step, read How to use manual partitioning during installation?

10. We are now ready to install! Click 'Install Now', and allow the partitioning tool in the Ubuntu installer to write changes. Now you should be prompted to select a time zone. The Ubuntu installer wisely tries to guess your time zone, assuming you have a functioning internet connection.

    

11. You will now be confronted with a choice of languages again. Select your language, and on the next step, select your keymap (keyboard layout). If you are unsure, click on the button to detect your keyboard layout.

    

12. On the next screen, you may set your username, display name, and password. Pick a strong password. A strong password can contain lowercase alphabetic, uppercase alphabetic, numeric and other printable characters. Although your username must be all lowercase, your display name is what is displayed in the login menu and may contain capital letters and spaces in it.

    

    • If you dual boot with Windows, you will be faced with an option to migrate documents. Pick the users and folders you want to migrate.

13. Now you may sit back with no trepidation, until faced with the prompt stating the installation is complete.

    

    • If you installed on a UEFI system you may want to run the following commands before booting into your new installation:

      sudo su;
      efi="/target/boot/efi/EFI"; \
      if [[ -e "${efi}/boot" ]]; then \
        echo "Backup:"; \
        mv -v "${efi}/boot/" "${efi}/boot-bak-$(lsb_release -cs)"; \
      fi; \
      echo "Copy:"; \
      cp -va "${efi}/ubuntu" "${efi}/boot"; \
      arch="$(ls -1 {efi}/ubuntu/shim*.efi)"; \
      arch="${arch#*shim}"; \
      arch="${arch%.efi}"; \
      cp -v "${efi}/boot/shim${arch}.efi" "${efi}/boot/boot${arch}.efi";
      

      This will copy Ubuntu's boot files to the default bootloader location and backup any previously existing files. By doing this you save yourself from headaches caused by non-standards compliant behavior of some UEFI firmwares found on VirtualBox as well as some older HP, Toshiba, Sony and other devices. Related bug report.

    • If you changed the drives' boot order in the BIOS during step #5 in order for the system to boot from the Live DVD / USB (or the system was configured already to boot from the DVD / USB drive) and click "Restart now", the system will reboot into the Live DVD / USB and not into your fresh installation. Remember to either roll back the changes you made in the BIOS or remove the medium (according to whether you want your current boot order configuration to stick or not) before booting again.

14. After you finish all these steps, connect to the internet, open the terminal (Ctrl+Alt+T) and run these commands:

    sudo apt-get update && sudo apt-get dist-upgrade  
    sudo apt-get install ubuntu-restricted-extras     
    

    This will update the packages on your system and install codecs for proprietary files with restricted copyright (MP3, AVI, MPEG, Microsoft fonts) and Adobe Flash Player.

Enjoy Ubuntu!

You are asking an interesting questions. I'll rephrase it my own words: Is it possible to run Ubuntu's installer, which is present at Ubuntu's LiveCD, from a regular Ubuntu system (installed to HDD)?

The GUI installer you found on the LiveCD is packaged as ubiquity. It's not clear, what will happen if you try to install and run this from your your HD install. While it might work, this does not seem supported or tested.

It's well known, that Debian-based systems such as Ubuntu can be installed in a chroot environment. Chroot is a tool, which can treat any directory as the root directory "/". You can easily create a chroot environment on your external HDD. If you want to boot this chroot Ubuntu, you have to install a kernel and a boot loader. While this is a known and tested method for Debian (and I have personally done this with Ubuntu), the tutorials on the web are outdated. Hence, this is only an option if you are familiar with grub and the Linux boot process.

Actual Answer: While your question is reasonable, it does not seem supported by Ubuntu or any other modern operating system. We have become so used to install OSs from a boot CD or USB, that other methods were neglected.

You didn't specify what the purpose of installing Ubuntu onto the external HDD is. If you leave this external HDD connected as it is, your computer will probably never boot it, because it prefers your internal HDD. If you change the BIOS settings, connect the external HDD to a different computer, or anything the like, then this (currently) external HDD can become the role of the boot HDD. Now, here comes the catch: It takes some guesswork to know now, whether and how this HDD will ever become bootable. The PC BIOS is a horrible bootloader, not designed for such things. That's probably why your idea is not supported through easy installation vectors such as Ubuntu's GUI installer.

For BIOS devices:

To install Ubuntu without CD/DVD or USB pendrive, follow these steps:

1. Download Unetbootin from here.

2. Run Unetbootin.

3. Now, from the drop-down menu under Type: select Hard Disk.

   

4. Next select the Diskimage. Browse to the directory where you downloaded the iso file of Ubuntu.

   

5. Press OK.

6. Next when you reboot, you will get a menu like this:

   

7. Select Unetbootin and you will get the "Try" or "Install" option there.

8. Finally, you can install your version of Ubuntu alongside Windows.

For UEFI devices:

• First, download rEFInd.

• Then, extract it.

• Now, follow the below steps to install rEFInd^[source], so that, you have a boot menu to boot to Ubuntu Live environment. Summary of the installation process given in the source is as belows:

  1. Open cmd with Administrator privileges.

  2. Then,

     mountvol S: /S
     

     • (you may change S: to any available drive letter)

  3. Type cd in cmd and then drag and drop the extracted folder to the already open cmd window. E.g.,

     cd "C:\Program Files"
     

  4. Copy refind

     xcopy /E refind S:\EFI\refind\
     

     • In this step S: will be the drive letter that you used above.

  5. Change directory to rEFInd

     cd S:\EFI\refind
     

  6. Rename config file

     rename refind.conf-sample refind.conf
     

  7. Note that {bootmgr} is entered as such; that's not a notation for a variable. Also, change refind_x64.efi to refind_ia32.efi on systems with 32-bit EFIs. Such computers are rare, and most of them are tablets. Check your Windows bit depth to determine which binary you should use.

     bcdedit /set {bootmgr} path \EFI\refind\refind_x64.efi
     

  8. Download EaseUS Partition Master Free and run it.

  9. Select your last partition.

     

  10. Right-click on it and from the context menu select Resize/Move partition.

      

  11. From the size shown beside Partition Size minus 4096 MB (to mimic a 4GB pendrive).

      

  12. Press tab and the rest will be auto-completed.

      

  13. Select the newly created partition and right-click on it. Select Create Partition from context menu.

      

  14. In this window, select FAT32 as the File System. Click on OK. Finally, click on Apply (present at the top-left of the window).

      

  15. Next, extract Ubuntu iso to this partition and finally reboot.

  16. You will be presented with a rEFInd menu. From here select the Ubuntu partition. It will boot into a Live environment. Continue with the installation. After the installation finishes boot to Windows.

  17. Run EaseUS Partition Master Free again.

  18. Right-click on the 4 GB partition and from the context menu select Delete Partition.

      

  19. Right-click on the partition above the Unallocated partition and select Resize/Move Partition from the context menu.

      

  20. Extend the bar to the extreme right by dragging the button.

      

  21. Click on OK. Finally, click on Apply (present at the top-left of the window).

      

  22. Open cmd with Administrator privileges.

  23. Then,

      mountvol S: /S
      

      • (you may change S: to any available drive letter)

  24. Remove rEFInd

      rmdir "S:\EFI\refind" /S /Q
      

      • In this step S: will be the drive letter that you used above.

  25. Reinstall grub as your primary boot manager

      bcdedit /set {bootmgr} path \EFI\ubuntu\grubx64.efi