sudo mkdir /mnt/casperx
sudo mount /dev/sdax /mnt/casperx

cp -r /mnt/original/* /mnt/casperx

sudo dd if=/path/to/casper-rw of=/dev/<partition>

sudo mkdir /mnt/casper-rw /mnt/target
sudo mount -o loop /path/to/casper-rw /mnt/casper-rw
sudo mount /dev/sdbX /mnt/target
cp -r /mnt/casper-rw/* /mnt/target