Windows will not boot and bluescreens after changing BIOS mode from ATA, SATA, RAID, or “Legacy” to AHCI mode

If you’ve installed windows on a machine and the BIOS was set to ATA, SATA, or RAID mode upon install and then switch the BIOS to AHCI mode, Windows will likely fail to boot and you will see a Bluescreen. This is because Windows 7 and other versions of Windows do not enable the AHCI driver by default. To enable the driver and resolve the issue, you can do the following:

To resolve this issue yourself, enable the AHCI driver in the registry before you change the SATA mode of the boot drive. To do this, follow these steps:

  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 one of the following registry subkeys:
    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci
    HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\IastorV
  5. In the pane on the right side, 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.

More details here from Microsoft: https://support.microsoft.com/en-us/kb/922976

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