gmirror problem

[Dimiter Petrov]

hello,
 i have freebsd 5.3 and i'm trying to make a software raid with gmirror.
 I hava a disk with data and i want to add another one to make a mirror.

here is what i am typing from man gmirror

    gmirror label -v -b round-robin data ad0

But there is an error 

   Can't store metadata on ad0: Operation not permitted.

I read the manual and try to add -h parameter to gmirror but there is still this error.

please let me know how can i solve this problem 

thank you