The IDE PATA HDD is the older type of drive with the 40 pin (80 cable) connector on the back. Some MB's now don't even have an onboard connector for them. When present, in the MB BIOS, there will be a setting to enable or disable the 2 IDE ports. If they are turned on, it will recognize a functioning PATA drive, if it's disabled, the OS won't see it.
I think your mean you have your SATA configuration is set up in AHCI (Advanced Host Configuration Interface) Configuration. If not please correct me.
These Intel SATA ports now come directly from the PCH (Platform Controller Hub), or an additional Marvel controller, whereas the older PATA drives usually come off a separate chip (like a B36x), that may control several other legacy features. So, like a light switch, they can be turned on or off, depending on needs.