Thank you, John & Tauno, for your answers.

> What you'll want to do is create a bootable CD with the linux kernel image
> and a a compressed root filesystem that will load into a temporary
> ramdisk (see "initrd"). This initrd filesystem will contain the needed USB
> drivers to allow the kernel to find your USB hard drive and remount that
> as the root filesystem once the kernel can recognize it.

I will give it (and me a try. Actually I would prefer to have the
kernel on a harddisk for easy kernel updates, but that's not so
important, a CD-RW would also do the job. (I'm out of Linux for some
years now, but in former times I compiled a new kernel approx. once a
week; may be times have changed here ?!)