Is Raid 1 possible with 2 x 3TB drives in XPS 435T/9000

Have a desktop XPS 435T/9000 with currently 0.5 + 1TB HDDs. Want to install two 3TB Hitachi drives with RAID1 to backup my work.  Installed everything and upon bootup the two drives only showed 750GB using RAID1, but I was hoping for 3TB!!!    Not sure why I was limited to 750GB?  Has anybody any clue why I was limited to 750GB. Is 3TB support on this dell model?   Spoke to Dell (Hong Kong) and they said they can RAID 2TB, but didn't offer much advice whether 3TB was possible.  Does this mean my 3T is not supported by the firmware or this dell model, even though only a couple of years old?  Any suggestions would be most appreciated.  Thanks