We faced booting problem on X7DBE-X when configuring the onboard SATA RAID with SATA DVD+/-RW (Model: LG GH22). We can't boot from DVD+/-RW if we form RAID using the onboard RAID of X7DBE-X. The issue is solved if changing the onboard RAID to Adaptec but Intel still not working. Please check to see if any solution as Intel can support RAID 5 but Adaptec can't.
This is the limitation for X7 platforms. If you want to use Intel RAID, you need to use IDE or USB ODD.