You still seem to be getting it wrong...
It's not "the BIOS on the motherboard" that does not support more than 2TB, it's a limitation of using the disks in MBR partition scheme, as opposed to GPT scheme.
If your system really is an RX300s7, then its motherboard definitely supports UEFI. But even if you set your system to UEFI mode, you will still be stuck to 2TB disk size, until you change the partitoning scheme from MBR to GPT.
Regarding the arrangement of disks, please try to re-read my last post. I'm not going to repeat it.
Creating 4 RAID6 disks out of 16 physical disks is just, er, hm, ...
With 50% redundancy waste, you'd better set up RAID10, which would give WAAAYY better performance.
Please don't get me wrong, but you might be well advised to get professional on-site help for the setup of your system.