<span style="background-color: #99FF99;">Thanks kl522,

that's not academic as I cannot boot KNOPPIX on my USB driver with the described tecnique. I wish I could.
Actually I can succeed copying KNOPPIX, linux and minirt.gz on a NTFS partition of my INTERNAL DISK the three files are fired with this 40_custom grub2 entry:

menuentry "KNOPPIX LXDE" {
set root=(hd0,2)
linux /KNOPPIX/linux boot=/dev/sda2/KNOPPIX/KNOPPIX ramdisk_size=100000 lang=it vt.default_utf8=0 nohome apm=power-off vga=normal norandr xmodule=vga nomce loglevel=1 tz=Europe/Rome
initrd /KNOPPIX/minirt.gz
}
no success on USB drive instead.</span>