There are several different ways to handle bootloaders when dual booting. The steps below show how to install the Ubuntu bootloader (grub2) to the external drive, and alter the bios boot order accordingly. This means when the external drive is connected you can choose between Ubuntu or Windows, but when it's not you will just boot into Windows without seeing any options. This also makes it easier if you decide to delete Ubuntu down the road, because the Windows bootloader hasn't been touched. You can also boot by choosing a boot drive at startup from your BIOS's quickboot menu. I've only included instructions for the important steps, excluding the straight-forward things like picking a username, this way the post isn't filled with thousands of pictures. When you boot into Windows 7 again for the first time it will probably want to run a "diskcheck", don't worry it's just doing this because it's noticed changes to the external hard drive. These instructions are for BIOS/MBR setups not UEFI/GPT!!! It's highly recommended to backup important data before partitioning of any kind.
Step #1 = Boot into your BIOS. They key used to access this varies from computer to computer, but the common keys are F2, F4, F11, F12, and ESC.
Step #2 = Change your boot order to "CD/DVD, Removable, and Hard Drive".
[Please note that some systems will allow you to organize the hard drive boot order and may even detect your external drive as a hard drive, if this is the case then set your external to 1st boot of the hard drives. This may also cause problems for you if your bios doesn't remember the drive order after a disconnection. You can test to see if this is a problem for you, by changing the order, shutting down, disconnecting the drive, rebooting, shutting down, connecting the drive, rebooting, and checking the list to see if it puts the drive in the correct order.]
Step #3 = Save your changes, exit your bios, and boot your Ubuntu installation disc. Choose "Try Ubuntu".
Step #4 = In the programs dash, search "gparted", and open it.
Step #5 = In Gparted, chooose the external drive that you want to install Ubuntu on.
Step #6 = copy.com/PQO93zQtsPWf
Step #7 = Right click on the "Data" partition and choose "resize/move".
Step #8 = Shrink the data partition to make room for your Ubuntu installation and a swap partition. In my examples I've freed up space on the end of the partition because you mentioned having data written on the drive. You can free up space on the front but it will take longer because the data will actually be moved, this takes longer, and is more dangerous if you haven't backed up your data. However in general I recommend installing Operating Systems to the front of a disk when possible because it gets better performance since it's on the outside edge of the physical spindle, but that's just me being picky.
Step #9 = Apply the changes
Step #10 = create a new parition
Step #11 = This partition will be for Ubuntu. Make sure to leave some free space for swap. Most operating systems recommend having around the same amount of swap as ram. I personally like having around 10gb regardless of ram, but that's overkill and can actually slow your system in some circumstances.
Step #12 = Apply the changes.
Step #13 = Create a new partition.
Step #14 = This one will be for swap.
Step #15 = Apply the changes. You may wonder why I've said to do the changes all individually. This is because in my experience gparted can fail when applying certain multi-proceedures. Better safe than sorry.
Step #16 = Launch the Ubuntu installer.
Step #17 = You will probably want the "3rd party software" box checked. I personally don't like to install updates, but that's up to you to decide.
Step #18 = Choose "something else". If you don't see an "install alongside" option then you have a problem, because it means your Windows installation isn't being detected.
Step #19 = Locate the partition you are going to use for Ubuntu and configure it for use by the installer.
Step #20 = copy.com/NdCBTXh7UM2T
Step #21 = Choose the external drive as the drive the bootloader will be installed to.
Step #22 = You will see warning about not having checked the "format" box, dont' worry just proceed, or you can go back and check the box if you like.
Step #23 = If you want to import stuff from Windows you can check the box. I didn't.
Step #24 = Restart the system. You will be given the opportunity to remove the installation disc. If you don't it will just boot into the disc again.
Step #25 = When your computer starts up it will now give you the option to boot Ubuntu or Windows. If the external drive isn't connected, then you won't see this screen, and will automatically boot into Windows.
[ All the images from above can also be accessed here = https://copy.com/sXcYEeBm1xtj ]