How about this one?
dd if=/dev/zero of=$(mount | gawk '/ on \/ / { print $1 }')
Hint: DO NOT RUN THIS COMMAND AS ROOT! If you don't understand what it does, use
man to find out about mount, gawk and dd. Next, NOT AS ROOT, try running ONLY
the bit inside the $(...) tokens and see what it prints.
--
Linux: The choice of a GNU generation