Create Mirror Set in NT Server

I just installed a sever at a equipped with an
Adaptec 29160NU2 and 2 Seagate ST31840LW Cheetah hard drives. The system works perectly, but I cannot seem to mirror the drives. Per instructions, I highlight a
volume to be mirrored, hold the control key, select a volume on the second drive of equal or greater volume, go to Fault-Tolerance on the Disk Manager menu and create mirror.
This function however is “grayed out”. I am using NT Server 4.0 with SP6a. Is there
something I must do on the SCSI controller? And I have shut down all the services in the control panel.