On 1 June 2007 I tested successfully a easy method to boot from the USB hard disk. There is the kernel parameter rootdelay, that gives the kernel time to setup the connection to the USB hard disk and therefore avoid that kernel panics occur. All necessary work to be done is in the file /boot/grub/grub.config. There the kernel needs to be called with the rootdelay parameter. The example below shows what has to be put into e /boot/grub/grub.config to get a 10s delay:

title=Gentoo Linux <version> rootdelay
root (hd0,0)
kernel /kernel-<version>-gentoo root=/dev/sda3 rootdelay=10

This solution is the preferred way, however it can happen that the kernel still does not boot. Therefore the next solution follows.