You first need to enable the Vista AHCI drives that are by default deactivated during the installation of Vista.
In order to activate the Vista AHCI drives, you need to go to the Registry of your PC and change some settings as described by Microsoft.
But, please bear it in mind that dealing with registry is a hazardous practice and may cause full or partial damage if not handled properly.
Before this, check the Guide from Microsoft:
How to back up and restore the registry in Windows
http://support.microsoft.com/kb/322756/
Then you may wish to proceed as follows if you want to enable the Vista-disabled AHCI drive on your Vista PC prior to set the SATA settings to AHCI mode in your BIOS:
To resolve this issue, enable the AHCI driver in the registry before you change the SATA mode of the boot drive. To do this, follow these steps (taken from Microsoft as link given below) :
1. Exit all Windows-based programs.
2. Click Start, type regedit in the Start Search box, and then press ENTER.
3. If you receive the User Account Control dialog box, click Continue.
4. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci
5. In the right pane, right-click Start in the Name column, and then click Modify.
6. In the Value data box, type 0 , and then click OK.
7. On the File menu, click Exit to close Registry Editor.
This is the full Microsoft guide on this matter:
Error message when you start a Windows Vista-based computer after you change the SATA mode of the boot drive: "STOP 0x0000007B INACCESSABLE_BOOT_DEVICE"
http://support.microsoft.com/kb/922976
And, Advanced Host Controller Interface specification for Serial ATA from Intel:
http://www.intel.com/technology/serialata/ahci.htm
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