There was a Linux Journal article on just this scenario, it should be on their website, I found it via Google.

On 1/23/06, Cleber P. de Souza < cleberps@gmail.com> wrote:
Hi.

I have a Centos 4.2 installed but the Raid 1 configuration wasn't set
during the installation process. How could I build the Raid 1 after
the system has been installed?

Thanks.

--
Cleber P. de Souza
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Howard Fore, howard.fore@gmail.com
"The less you know, the more you believe." - U2, Last Night On Earth