booting from usb drive

[oliver oli]

hello,

i installed ubuntu on an usb hard disk, but i don't what i have to do 
that the kernel can mount the root partition.

the usb drive is detected by the laptop, it boots GRUB and GRUB loads 
the kernel, but then i get a kernel panic.

i booted Fedora 2 successfuly from this hard disc. there i had to modify 
the initrd.img (howto at http://www.simonf.com/usb/). ubuntus initrd.img 
looks completly different, so i have no idea where to start.

it would be nice if usb booting from external hard drive could be added 
to the kernel (or maybe it is possible and i just don't know how).