ok, because not say "boot SDcar o another SD" only its possible boot from pendrive - hd-usb - network or SSD internal. If put SD card into SDreader expansion or multireader, not is possible make boot.
well i think for those system with the hard drive or maybe those with windows xp preinstalled you can do this because I have tested it on mine which is model A150 and both slot does not show up in the boot menu even though I have an SD card slot in there..