Exactly, I just built a RAID without deleting the old config. and its accepted. I am wondering how is successful. Although it was just for testing purposes. By the way, thanks for your reply.

Mandatory because until you delete the existing RAID configuration, the drives will show as being part of an array and will NOT be available to add to a new configuration. Delete the existing, then create (and initialize) a new one.

Why do you ask?

I suppose it may depend on which RAID controller it was, how it was configured, and/or how you configured the new one ...