Answer

This issue occurs if the AHCI driver (Msahci.sys) in Windows Vista is disabled. The AHCI driver must be enabled first (Windows environment) before enabling AHCI mode in the bios. To do this, please follow the given procedure step-by- step:
(1) Go to your BIOS and disable AHCI mode.
(2) Under Windows Vista environment, go to HKLM\SYSTEM\CurrentControlSet\Services\msahci\.
(3) Edit the "Start" key and change its value to “0”.
(4) Then reboot the system, go to your BIOS and enable AHCI mode. Save and Exit. Upon the next boot up, Vista will detect the change, install the driver and might ask for a reboot.