Xplorer4x4,
"If memory serves..." ?!? You're trying to solve a technical quandary, but haven't provided the BIOS manufacturer, the OEM if applicable, the BIOS version, etc. This would go a long way to assist other user's to look into the issue and figure out ways to help. Nevertheless, working with the information you did provide, selecting the options in the first set of items (RAID or AHCI) would seem to specify which mode (BTW, "architecture" may be a more descriptive way of thinkng of it, though it is not likely the manufacturer would say that as they have to use discrete meanings) you wish the computer's hardware layer to handle or treat the drives. If you had two drives, RAID is a valid option (incidentally, RAID would most likely work with just one drive, though pointless--at least I can do it with my BIOS), and would be necessary to implement a RAID setup. Otherwise, your would select AHCI--no array. The second set of options would, using a bit of assumption, is selecting the compatibility mode that the storage controller uses when addressing the drives. While it would be unlikely you would be able to select RAID compatibility addressing if you already selected AHCI mode, it is theoretically possible--the BIOS may be able to handle the complexities, but again, I doubt it. So if you selected RAID mode for the controller, then the logical selection for the drive addressing is RAID. If you selected AHCI mode for the controller, then the performance and advanced features of AHCI would be desirable, so you would select AHCI for drive addressing. So why is IDE an option? For compatibility. It is possible that your BIOS has extended capabilities, but short of that, being able to select the addressing mode allows older OS's to use a very modern drive--the BIOS will operate in AHCI, but to the OS it will appear as if the IDE drives it can handle are present. OS's are just one example. (BTW#2, IDE is remnant term for use in this context as both PATA ("IDE") and SATA drives have the electronics on the drive unit itself, as opposed to a more complicated system that was in use before the drives electronics became integrated, but I digress).
Now as to why I would think the first set would dictate the second set and not vice versa. If the second set was the mode (drive architecture), then selecting IDE would mean the drive addressing options would need more than AHCI and RAID. It would present the architecture as IDE, but the drives would be expecting AHCI addressing. This is why it is probably the way I described above.
I take responsibility for my post's accuracy, but with the caveat I am working with limited information. Perhaps next time you could shutdown and enter the BIOS to make sure of your "memory serves"--and while you're there, get the manufacturer, the version, etc. The details are helpful in deciphering this.
-FM