Comment 3 for bug 331327

Although the "bs=%sM" is wrong and will certainly confuse dd, it's not the source of this particular crash. It is

  'count=1' % persist

I think this line would make most sense:

  popen(['dd', 'if=/dev/zero', 'bs=%s' % persist, 'of=%s/casper-rw' % target, 'count=1'])