I will answer to my post
I found the solution after hours of web browsing.
The solution is simple, but difficult to find
My mistake was that I imagined the RAID system of my motherboard was acting as RAID system I met before, and should be used the same way to install a Linux distribution.
I mean create an array from RAID manager and give it to the distrib.
This is not the way CentOS wants it, and I think this comes for the RAID manager directly, the way RAID1 is managed on my motherboard.
Solution was to leave the distrib manage all the stuff.
I mean I let all my HDD visible as normal/classical HDD and I had to create RAID arrays from the SW RAID manager of Linux.
This is done quite easily with the installer:
* in HDD selections screen of the installer, check all the HDD you want to use for the RAID arrays to create and choose to manage partitions creation manually:
=> in my case I checked 2 HDD to have RAID 1 arrays: I wanted to have one RAID1 array for the system /, one other RAID1 to mount on /var , and one more for swap
* remove all mount point that you will see on the left part of the partitions definition window if you have any
* create a new entry point ; you will have to give name, mount point and size
* then you will be able to select RAID 1 as file system in the window to define the entry point
Installer will manage to create all what has to be created to reach size and RAID1 availability at boot (if your partition contains boot area)
The installer will also take care to install the loader as it has to be installed.
That's alll. I have tested it several times and it was working in all cases.
Hope this may help someone. This is not related to ClearOS but to CentOS. But as I have posted the question here, I wanted to answer it here also.