Downgrading Windows 2008 to 2003

Hi:
Not sure what you are attempting to do, but if you goal is to have Server 2003 as your OS, you will need to install it from the origonal media.

If the goal is to add them to a new/different RAID controller, there are only two ways that I know of.  One is to move the OS with a hardware independent restore product such as Shadow Protect, the other is to introduce the controller into a working system and add the drivers for that controller, then move the controller and the drives to the new system (or to attach the drives to the controller), powering down at each step of course.

I have no problem with drivers. I can see the RAID-5 disk in windows 2003, but cannot reactivate it. If I start the computer with Windows 2008 R2 then there is no problem to connect to the disk. I orginally made the disk with Windows 2008 R2, but now I want to use it with WIndows 2003 on then same PC.

The RAID-5 disks are only data disks, no OS is on them. The OS are on standalone basic SATA-disk.

Windows 2003 on one basic disk and Windows 2008 on one basic disk.
I just disconnect the Windows 2008 disk and connect the Windows 2003 to swap operating system.
I did setup both OS on the same computer. Both OS works fine, except Windows 2003 cannot reactivate the RAID-5 disks who where created with Windows 2008. Hope you understand.

Ok, that is more clear.  Boot on the Server 2008 OS, backup/image your data from the RAID 5 array, remove the array,  boot on the Server 2003 OS, create the array with the 2003 OS, restore the data.

Then mirror your OS drive.

I know I can do that, I just want to know if it is possible to use RAID-5 data disk created with Windows 2008 on Windows 2003

What is the error you are getting?

Here is the error  

Forget event error..  

do you have data that is important already on these disks?

Yes some  the RAID is 4 x 2 tb