Can the unionfs data be written to HDD directly through mount of HDD
mount your hdd (let's say /mnt/sda1/ )
create some dir where you want to put the overlay (/mnt/sda1/union)

add the branch
Code:
unionctl /UNIONFS/ --add /mnt/sda1/union/
make the ramdisk read-only to unionfs
Code:
unionctl /UNIONFS/ --mode /ramdisk ro
note:
/home and /tmp are not mounted in the union, the still point diretly to the ramdisk
(for whatever reason - maybe someone can clear this up for me)