 How to enable AHCI mode for a computer where an operating system had already been installed It is quite possible that AHCI mode is already enabled in your PC. Go to Device Manager, open the section IDE, Add to a TAPI Controllers. If you just enable AHCI mode for after-windows installation, you will face Absod Era, Blue Screen of Death, or your system will refuse to boot, getting stuck in a series of stunts. That is why it is recommended to enable AHCI before installing Windows. Run Registry Editor and type in the command Raggedit. Go to the section Start Overwrite, set the zero value, restart the computer and go to UFI Obias. In UFI Obias, find set as settings to select the mode for memory devices. Switch them to AHCI.