TechNucleus

AHCI(Advanced Host Controller Interface) mode brings 3 main advantages:

1. Supports NCQ (Native Command Queuing) allowing SATA drives to accept more than one command at a time and dynamically reorder the commands for maximum efficiency.
2. Supports hot plugging of devices.
3. Supports staggered spin ups of multiple hard drives at boot time

Normally it's default to IDE (at least mine). When you install OS without enabling AHCI in the BIOS and enabling after installation causes your OS unusable. Most people have been advising to reinstall Windows if you want AHCI enabled.

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:

1. Click Start, type regedit in the Start Search box, and then press ENTER.
2. If you receive the User Account Control dialog box, click Continue.
3. Locate and then click the following registry subkey: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci
4. In the right pane, right-click Start, and then click Modify.
5. In the Value data box, type 0, and then click OK. Note: Note down the actuall value if in case you want to revert back.
6. Close Registry Editor.

After this you’ll have to restart your computer, go to BIOS and enable AHCI. When you log in to Windows again, you’ll notice the installation of drivers for AHCI. Another restart will be required to finish the driver installation.

I also found this link useful.

Disclaimer:

I am not responsible for anything if your system crashes or not working. You are responsible for your own mess with the registry. Take appropriate backup before make any changes and Dont' blame me later on.

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