[ubuntu] Install Ubuntu on a USB hard drive without changing the MBR

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http://ubuntuforums.org – I have downloaded an ISO file containing the installation disk for Ubuntu 12.04. I burnt this onto a disk and am hoping to be able to boot from it. Now what I want is kind of a plug-and-play system. I want to install Ubuntu on an external USB HDD, such that my computer automatically boots Ubuntu when the USB drive is connected during PC startup. To me it is no problem if I get a GRUB-like screen to pick my OS, but I want to see that only when my USB drive is connected. Is this possible, and if so, how? (Hardware)