Advanced Host Controller Interface - Wikipedia, the free encyclopedia
Many SATA controllers offer selectable modes of operation: legacy
Parallel ATA emulation, standard AHCI mode, or vendor-specific
RAID. Intel recommends choosing RAID mode on their
motherboards (which also enables AHCI) rather than the plain AHCI/SATA mode for maximum flexibility, due to the issues caused when the mode
is switched once an operating system has already been installed.
[2] Legacy mode
is a software backward-compatibility mechanism intended to allow the SATA controller to run in legacy operating systems which are not SATA-aware or where a driver does not exist to make the operating systems SATA-aware.
I've wondered this too. I was just looking at a bios yesterday thinking what would happen if I switched it to raid or ACHI with an OS already installed.