Hi Logic,
Need a bit more info.
What model Thinkserver?
What is your "current raid"? What controller is it on? Have you added the physical drive to the existing RAID set and extended it? or have you Added a single drive?
I'm not fully familiar with ThinkServers but after adding the drive then you will most likely have to configure a new volume in the Array/RAID manager usually during bios/boot up.
Are you able to provide any screenshots of your bios/raid config along with the specs of the thinkserver and your current raid config, which controller it is on and steps you have taken after simply inserting a new HDD?
Chris