Allow Gigabyte GA-EP45-DS4 to boot up with AHCI enabled

If your computer is failing to boot and hangs during AHCI enumeration with a possible code 23 in the top right corner, and it only happens when (1 or more) SATA HDD are connected that have a MBR (Master Boot Record), this may fix it for you, as it did for me:

• Turn off computer.
• Disconnect (not-really-)faulty HDD.
• Turn on computer.
• In BIOS, disable AHCI:
• BIOS > Integrated Peripherals > SATA AHCI Mode = Disabled
• Save changes and turn off computer.
• Reconnect (not-really-)faulty HDD.
• Turn on computer.
• Boot in to operating system of your choice. e.g. Linux
• Open (not-really-)faulty HDD in a hex editor. e.g. hexcurse /dev/sda
• Set the 3 bytes starting at offset 0x1C3 to FE FF FF, save and quit.
• You may wish to make a note of the original values you're replacing in case you need to undo.
• If using Arch Linux, add ahci to mkinitcpio.conf modules list and rebuild kernel image
• Reboot, in bios enable AHCI again, save and reboot

There is an error in the BIOS/AHCI controller ROM that has been reported to Gigabyte, but whether they've released BIOS updates for all affected motherboards is a mystery. For example, there's a "beta" bios update for my motherboard, but without a changelog I'm hesitant to install it.

References:
http://forum.giga-byte.co.uk/index.php?topic=8585.0
https://wiki.archlinux.org/index.php/AHCI
http://forums.tweaktown.com/gigabyte/42356-removing-ahci-option-rom.html
http://forums.tweaktown.com/gigabyte/28441-gigabyte-latest-beta-bios-2.html