The easiest way to find out is to shut the computer down, disconnect power from one of the raid drives, then boot into the bios and and change over to AHCI. If this was a bootable raid, then make sure you set the remaining raid drive as the boot drive. If you can boot properly, all should be good. If not, shut down, reconnect the other raid drive, boot into bios and switch back to raid.
If you managed to successfully turn off raid and boot ok, you can then use the disconnected drive as a backup since it's a mirror, or you can reattach it, format it, and use it as a storage drive.
As a precaution, it's always best practice to backup your drive/data before changing drive controller settings.