You need to change the mount command to reflect the filesystem for the usb drive with the rootfs.img from vfat to ntfs
also remember to check the drive letters in gparted and change them as needed.
mount -t ntfs /dev/sdb1 /mnt/usb
USB to sata cables are very iffy with this script. You might want to try the 2.0 guide linked at the top of the guide on page 1
http://community.wd.com/t5/My-Book-Live/GUIDE-Debrick-MyBookLive-v2-0/td-p/567582
