A new HDD should be AHCI - and it should just install right into the system. Boot priority might need to be set to SSD first, then HDD. Other than that, install, partition using Computer Management > Storage, format the drive and assign the drive letter. You might want to move your personal folders from the SSD to HDD. This is easy, just go to your libraries, right click Documents, Music, Pictures and Videos, choose properties, then assign a folder to the HDD. If multiple users log onto the computer, and you are worried about security, you will need to assign security to the new folders.