Description of problem:
if you dd an iso to the usb and then later use livecd-iso-to-disk to overwrite it the iso9660 signature is still present (offset 8000?) so tools like blkid think it is an iso.

I need to do one of the following
 1. just wipe the first 1M using dd if=/dev/zero bs=1M count=1 of=device
 2. detect that the blkid thinks it is iso9660 instead of extX or vfat

I'm leaning towards doing #1 when we do a --reset-mbr since that involves wiping everything anyway there is no chance for data loss.