In the order details for your Service Tag, the only RAID controller listed is the onboard S110 RAID controller.  The S110 is the lowest-end controller you can get.  IF that is the controller you are still using, then the problem is your controller only supports a maximum of 4 disks, AND it does not support RAID 6 (only 0,1,5,10). You should be using the H700 for a storage system like that anyway (or the H310 at a minimum). If you do indeed have a higher-end controller, then you are most likely missing the second SAS cable from the controller to the backplane.
I purchased 7 additional 2TB drives for my Dell PowerEdge T620, which shipped with just 1 such drive almost a year ago now.  My model has the 8 HDD bays in front and after installing the 7 new drives I note that only the four drives in the top row of bays are recognized by the system/BIOS.  I'm running Ubuntu Server 12.04.4 and issuing 'fdisk -l' lists only the four drives, as well (sda,b,c,d).  I'm hoping to set the drives up in a RAID 6 configuration if I can get the system to see eight -- is this just a simple BIOS setting change?  Please tell me I don't need another controller or the like?!  If it helps on the Dell Support side of things, my ST is [removed per privacy policy].  Regards, Mark D.
Dell T620 only recognizes 4 of 8 installed HDDs