How to enable AHCI in Windows 7 after installation as ATA

To enable AHCI (Advanced Host Controller Interface) after Windows 7 is installed with ATA mode, do the following:

• Start Windows 7
• Run Registry Editor
• Go to: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci
• Change the “Start” value from 3 to 0
• Reboot and go to the BIOS immediately
• Set BIOS to AHCI
• The next time Windows 7 loads, it will auto-detect the AHCI setting and install the drivers/devices it needs for proper AHCI operation.

Note: eSATA drives require SATA Operation in the BIOS be set to AHCI for proper eSATA operation.

Reference

• How to enable AHCI in Windows 7 RC after installation
• Windows 7 and AHCI
• GUIDE – Enabling AHCI mode AFTER Windows 7 Installation
• Error message when you start a Windows 7 or Windows Vista-based computer after you change the SATA mode of the boot drive: “STOP 0x0000007B INACCESSABLE_BOOT_DEVICE”