Ubuntu 11.10 is on external HDD, how to boot (using grub4dos), and which initrd and vmlinuz to use?

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http://askubuntu.com – I installed Ubuntu 11.10 on external HDD. How can I boot it? I used grub4dos before, and it worked with Fedora distributions. Problem I have now is that I don't know which initrd and vmlinuz files to use. Tried with one I have in /boot directory, but booting did not worked... Can you give me some ideas what to try, do i have to use different initrd and vmlinuz, or I can solve this with other solutions. Also, just to mention that boot goes to BusyBox v1.18.4 command prompt. Thank you in advance! (HowTos)