Mount in Freebsd 7.0

[Juliano Romani]

Hi...
I have a embedded system based in pfsense structure. When it boot, / 
dev/ad0a is mounted in / and a memory disk (/dev/md0a) is mounted in / 
mnt. Then i chroot /mnt. Now, i need mount /dev/ad0a in /cf, but it  
only possible in read-only mode. I need it work in read-write mode. In  
freebsd 6.3 it works, but now, i try in 7.0 and this happening. Any  
ideias about happen???

A example of real case, FreeBSD 6.3,:
# mount
/dev/ad0a on / (ufs, local, read-only)
devfs on /dev (devfs, local)
/dev/md0c on /mnt (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0a on /cf (ufs, local, read-only)
# mount -uw /cf
# mount
/dev/ad0a on / (ufs, local, read-only)
devfs on /dev (devfs, local)
/dev/md0c on /mnt (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0a on /cf (ufs, local)
#

FreeBSD 7.0:
# mount
/dev/ad0a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/md0c on /mnt (ufs, local)
devfs on /dev (devfs, local)
# mount -o ro /dev/ad0a /cf
# mount
/dev/ad0a on / (ufs, local, read-only)
devfs on /dev (devfs, local)
/dev/md0c on /mnt (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0a on /cf (ufs, local, read-only)
# mount -uw /cf
mount:  : Operation not permitted
#

remember that /cf is mounted inside of chroot.

Thanks