It is popular problem as Intel Matrix Controller see only 4 disks if RAID is enabled - all You need to do is enabling RUN AHCI ROM in BIOS. You can find it in BIOS under:

-> Intergrated Peripherals -> On-Chip IDE Device

Set Run AHCI ROM to Enabled (Enabled = 6 drives / Disabled = 4 Drives)

It should solve Your problem.