New
#470
The manual for the board itself will show what options are present. Generally you don't go into the bios setup and see a separate AHCI tab. You have to get familiar with the layout and hard drive options and go in further to find out just where that option if present is located.
Here I have the feeling the board saw that or another default drive setting enabled since the initial clean install of 7 when new wouldn't load up! When going back into the bios setup and switching over to the Native ide mode 7 then loaded right up until just lately changing to AHCI where 7 automatically adjusted to it.
Still the best place for bios settings would be in the user manual or support site's documentation for that make and model board. That's what you will need to go by anyways.