#!/bin/bash

d="$1"
n="$(sudo dumpe2fs "$d" 2>/dev/null | sed -ne '/^Block count:/ {s/[^0-9]*//;p;}')"
echo "$d $n"
echo -e "P1\n1000 $((n/1000))" > /tmp/usedblocks.pbm
sudo debugfs -R "testb 1 $((n-1))" "$d" |\
sed -e 's/.*marked in use.*/1/; s/.*not.*/0/;' >> /tmp/usedblocks.pbm

eog /tmp/usedblocks.pbm