The operating system (and installer) will only detect the LOGICAL drive(s) presented by the RAID controller. My guess is that you configured the array to be RAID1. If that is how it is configured (go to the BIOS to find out), then all is as it should be.
By design, the RAID controller hides the physical properties of the RAID. Dell has some add-on software that you can run from windows that drills inside of the controller & shows physical disks, and their health.