Can you check whether you have a PERC controller on the server. If you have a PERC controller installed on slot 1 of below link then you will be able to use all 8 slots on the back plane. If not you have software RAID controller S110 and S110 only support upto 4 SATA drives. [URL] 
I have a PowerEdge R520 that we are trying to upgrade from using 4 hard drive bays to 6. The BIOS only recognizes the first 4. They are all SATA drives, and there is 1 cable from the motherboard labeled SATA A-D going to SAS-A in the backplane. There is an Empty SAS-B on the backplane. The only other SATA labeled space on the motherboard is taken up by the DVD drive. The Owner's Manual states in the Technical Specs: Up to eight 3.5 inch or 2.5 inch, internal, hot-swappableSAS, SATA, SAS SSD. SATA SSD, or Nearline SAS harddrives in hard-drive slots 0 through 7.  Do we just have the wrong cable or configuration, or does this server not really support more than 4 SATA drives? Thank you for your input.
PowerEdge R520 RAID and SATA Drives