Unfortunately all tutorials I have found online until now on how to setup software-raid-1 are outdated on ubuntu 12.40

My target is to setup it on a system with a secondary disk drive that is already running. Format is not an option!

I am trying to follow and adapt from 11.10 to 12.04 the following tutorial: http://www.howtoforge.com/how-to-set-up-software-raid1-on-a-running-lvm-system-incl-grub2-configuration-ubuntu-11.10-p2

On the above tutorial there is a successful command which creates a raid-1 array by setting the first disk drive with the installed system as missing: mdadm --create /dev/md0 --level=1 --raid-disks=2 missing /dev/sdb1

But when the time comes to add the first main drive with the installed system on the raid-array with this command: mdadm --add /dev/md0 /dev/sda1 I receive an error message.

The error message says that the device /dev/sda is (which makes sense) busy!

Note: hardware raid solution is not available since the system is a laptop with two disk drives!

Thank you