Well, I just couldn't give up.
Grabbed myself a copy of GParted Live CD, created the partitions manually (2GB for system, the rest for storage), copied the KERNEL and the SYSTEM files from the USB to the system partition in the hdd, and finally created my extlinux.cfg file pointing at the kernel and the respective "boot" and "disk" partitions. So far so good, the system is running. I only had a minor "problem", where the pop-up onscreen keyboard froze my machine for a moment. But all in all, I am indeed satisfied.
Best Regards,
D.