I am unable to online new hard disk in dell pe 2950 server

Hi,
i am netwok administrator and using windows server 2003 wiht dell 2950 server i have already configure raid5 and now i want to add new hard disk when i go to ctrl+r i can see my new hard disk and status is ready but i can not online this when i am trying to online the message is comming operation failed can any body help me regarding this thanks in advance,
regards,
itbar gul