Final answer:
Enabling AHCI on a computer system with Windows OS already installed can cause issues, such as blue screen errors or failure to boot.
Step-by-step explanation:
If you turn on AHCI (Advanced Host Controller Interface) on a computer system where Windows OS is already installed, it can cause certain issues. AHCI is a mode that allows the operating system to access advanced features of SATA (Serial ATA) drives. When you enable AHCI after Windows is installed, it may not recognize the hard drive correctly and lead to a blue screen error or prevent the system from booting.
To enable AHCI without encountering any issues, it is recommended to make the change before installing Windows OS. However, if AHCI is enabled after installing Windows, you can try updating the drivers for the SATA controller in Device Manager. If that doesn't work, you may need to reinstall Windows with AHCI enabled.