Thank you Robbieduncan. the thing is that I obtained 4x 4TB enterprise SAS drives and installed them in my Mac Pro 2008 but they don't appear and the discs are not even spinning. After a google search I found that I need an Apple raid card to use them as a RAID, but I don't need the raid option, I only want to use them as storage separately.
From what I read in the apple support page, the motherboard does support SAS drives. I also found this script thingy that supposedly helps.
This article contains a script used to configure Promise RAID Controllers and SAS-connected Expansion Chassis for Direct Attached Storage.
But I have absolute no idea how that works.