freebsd stable - Dec 2010 - boot0cfg -B can't work

I want to rewrite boot0 into mbr but failed, infos follows:

# boot0cfg -B ad2
boot0cfg: write_mbr: /dev/ad2: Operation not permitted

# ls /dev/ad*
/dev/ad2     /dev/ad2s2   /dev/ad2s2b  /dev/ad2s2e  /dev/ad2s2g
/dev/ad2s1   /dev/ad2s2a  /dev/ad2s2d  /dev/ad2s2f  /dev/ad2s3
> uname -a
FreeBSD bsd.hasee.cpu 8.2-PRERELEASE FreeBSD 8.2-PRERELEASE #0: Sun
Dec  5 19:14:33 CST 2010
root@bsd.hasee.cpu:/usr/obj/usr/src/sys/HASEE  i386

-- 
Regards,
Yue Wu

Key Laboratory of Modern Chinese Medicines
Department of Traditional Chinese Medicine
China Pharmaceutical University
No.24, Tongjia Xiang Street, Nanjing 210009, China

On Sun, Dec 05, 2010 at 04:29:24PM -0800, David Wolfskill
wrote:
> From boot0cfg(8):
> 
> NOTE
>      Protection mechanisms in the geom(4) subsystem might prevent boot0cfg
>      from being able to update the MBR on a mounted disk.  Instructions for
>      temporarily disabling these protection mechanisms can be found in the
>      geom(4) manpage. Specifically, do a
> 
>            sysctl kern.geom.debugflags=0x10
> 
>      to allow writing to the MBR, and restore it to 0 afterwards.
> 
Thanks, I remembered that boot0cfg in the previous version can do it
without changing the sysctl parameter, maybe it's changed in newer
version.

-- 
Regards,
Yue Wu

Key Laboratory of Modern Chinese Medicines
Department of Traditional Chinese Medicine
China Pharmaceutical University
No.24, Tongjia Xiang Street, Nanjing 210009, China