livecd / # du -sh /mnt/initrd/
31M     /mnt/initrd/
livecd / # rm -R /mnt/initrd/*
livecd / # cp -R /other/* /mnt/initrd/
cp: writing `/mnt/initrd/bin/insmod': No space left on device
cp: writing `/mnt/initrd/bin/pivot_root': No space left on device
cp: writing `/mnt/initrd/bin/loadkmap': No space left on device
cp: writing `/mnt/initrd/bin/umount': No space left on device
cp: writing `/mnt/initrd/bin/basename': No space left on device
cp: writing `/mnt/initrd/bin/busybox': No space left on device
cp: cannot create special file `/mnt/initrd/dev/vcs35': No space left on device
cp: cannot create special file `/mnt/initrd/dev/vcs36': No space left on device

(says this about 50 more times for other files)

livecd / # ls /mnt/initrd/
bin  dev  linuxrc  sbin


(... but some files are copied, so its not ro)