Abu-saeed,  It sounds like the issue may be that the raid driver isn't being loaded. How are you trying to load the OS, by booting to the 2008 media? Depending on the controller you will have different driver files. If you have the H310, H710, or H710p then this is the driver you will need -  [URL] If you are trying to load the OS through the F10 Lifecycle Controller, then this is the file you will need - [URL]  Let me know if this helps.  
I have 2 Servers PowerEdge R720, and I am trying to install windows server 2008.  I used the windows drivers, and when i got hard dive screen i did not it   please i need help
PowerEdge R720 - Could not detect Hard Drive