I have a DP55WG that successfully boots without a display adapter in AHCI mode.
If, however, I change to RAID, it will stop booting at E7 on the onboard POST code indicator.
Is there anything I can do to address this? It's already on the latest BIOS at downloadcenter.intel.com
I have disabled the beep option in the BIOS for no graphics card.
I understand that your system will not go thru POST if it is set to RAID mode and there is no display connected.
This has been reported in the past and it is because the RAID controller will freeze if it is not able to send a display to a monitor. This is usually a normal behavior as if a problem is detected on the array, booting the system like this may corrupt the information.
The only work around that I may think of is to install a low end video card on the system and not installing a monitor to it.