Which RAID controller are you using?  I would assume you are using the PERC H710, which does NOT support non-RAID, so in order for anything to be presented to the OS, you will need to configure it in RAID first.  If you don't want to use RAID (even single-disk RAID 0's), then you will need to get another controller.  The PERC H310 supports non-RAID (the other controller options for the R720 do too, but they won't run Linux).
Hi,     I have this R720 server I want to install CentOS 6.3 on. It has two 256GB partitions which were previously configured as a RAID-1 volume. I removed this because we do not need RAID and just need performance when it comes to disk read/write. However, the CentOS installer fails to recognize the two physical disks. But if I configure the disks on RAID-0 the the installer recognizes the disks. Is there anything I need to do in order for the installer to recognize the physical disks? Thanks. Duminda
Installing CentOS on Dell R720 after disabling RAID