[CentOS] CentOS 4 Software Raid1 questions

I want to mirror an existing 40GB data only drive using
software Raid1 on my new CentOS 4 server. The existing
drive is connected to a Promise Ultra 100 TX2 controller
(non-raid). I have read about mdadm and understand how to
create the Raid1 on /dev/mdxx devices. However I would like
to know if the existing data on the orignal 40GB drive in
the system will be destroyed when I create the raid with
mdadm. Also do the 2 drives after being setup with mdadm
have to be reformated? Both the 40GB drives are now
formated with ext3. If not, will the added 40GB drive
assigned to the Raid1 atomaticly sync to the original 40GB
drive. If so, what do I need to do to assure this will
happend automaticly? Your comments are appreciated, as this
is my first time to setup a software Raid1. 
 
Lee
 

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