Weird screen when I attempt to boot Ubuntu from a usb hard drive

I tried to install Ubuntu to an external hard drive (WD My Book Essential 1.5TB). The live CD loaded fine, and I choose custom install. I made 2 primary partitions for Root(/) and Boot(/boot) and a logical partition for the swap space. I'm pretty sure I installed GRUB into the external drive. I then booted into windows 7 and configured the rest of the space left on the drive as an NTFS partition to later backup my Windows 7.

However, when I tried to boot Ubuntu from the external drive I got a black screen with green lines going down. In the green lines were (what looked like) percentage signs. I assume this is fairly rare because its never happened to me before. My computer wouldn't do anything from there, and I had to force shut-down. This happened twice, and then I went back to Windows. The NTFS partition still works fine, so I doubt its the drive.

Does anyone know what I can do to fix this or what mistakes I may have made during installation? Any help would be appreciated. Thanks

Edit: I got it so that the screwed up screen is gone but now it's just a blank black screen and no boot. Also, I tried using the GRUB menu, but it doesn't seem to want to come up no matter what I do or what keys I press. Could it be a problem with the location GRUB is installed to?

I don't know about the percentage signs pops up in your screen, but I had a graphic issue when I tried to install Ubuntu 12.04 on Dell Inspiron 1501 from USB-thumb drive. As soon as I booted in Ubuntu12.04, I got green, yellow, and red lines going down.

I solved it by using the nomodeset kernel boot option.

Since you have installed Ubuntu on your external drive you might try fixing your problem as follows:

Boot to your external drive.
Hold down Shift to get to the GRUB menu.
Press e when GNU GRUB pops up.
Delete quiet splash and type nomodeset in its plac.

Press Ctrl+X and see if Ubuntu boots normally without any graphic issue.

The above is a temporary fix. It won't carry over, the next time you boot. To make it permanent, so it works every time you boot:

In the Terminal (Ctrl+Alt+T), run gksu gedit /etc/default/grub. This opens GRUB's main configuration file in a graphical text editor.
In the editor, find the line that starts with GRUB_CMDLINE_LINUX_DEFAULT.
On that line, delete quiet splash and add nomodeset.
Press Ctrl+S to save the file, then quit the text editor.
Back in the Terminal, run:
```
sudo update-grub
```