You can add another SATA HDD for storage without adding it, or otherwise modifying your existing RAID volume. So you've connected the SATA data cable from the board to the HDD and you've connected a SATA power lead from your PSU to the HDD, and it won't boot?
-- Did you try using another SATA cable?
-- The other SATA port?
-- Another SATA power lead from the PSU?