Re: Ubuntu v12 Installation on a external HDD issue
I had a similar problem, where one laptop allowed me to boot into the external USB HD and another laptop would not. I knew the install on the external HD was OK, because it worked with the first machine.
In my case I had a booting linux partition on the internal drive; the solution was to install PLOP, a boot manager, to switch from the sda drive to the sdb drive from the GRUB2 menu. Probably doesn't help you though, if you can't modify the sda drive. Probably you can make a "boot CD" with the boot files on it, plus PLOP, but I cannot give you full instructions.
Hope that helps you find a way forward!
Sony vaio vgn-fw235j; Synaptics touchpad; triple boot: mint 12 64-bit; Precise 64-bit & Saucy 64-bit. 2TB external USB drive with more...