Dec, 2019

By running my SSD on a SATA controller in RAID mode instead of AHCI. I had thought RAID mode for a SATA controller supported all the advanced features of AHCI though.

if your benchmark is close to what the specs indicate when NOT using Rapid Mode.
Intel X79P motherboard
On X79P motherboard I have to choose "RAID" mode if i want two drives to be in RAID regardless of what the rest are running at (which I assume is AHCI).
In general choose "AHCI" if you don't need to setup RAID for any drives. No need to change it now though if your performance is fine.

AHCI is a close cousin to RAID and running in RAID mode will result in a very small performance loss.
Unless you need RAID, always run in AHCI mode.

AHCI and RAID are different modes; however, I would expect that RAID drivers tend to use as much of the AHCI features as possible.
RAID vs ACHI modes
Several years ago mobo makers caused confusion on these points. Some provided drivers for AHCI devices and RAID devices separately, and that was clear. You only set the SATA Port Mode to RAID if you actually were using RAID arrays. But others combined the two drivers into one, so you HAD to set the port mode to RAID to use that driver, even if you were NOT using RAID.

These days, however, the drivers for AHCI devices are "built in" in Windows Vista and up, so there is no need to install such drivers separately. Thus, there ought not to be any unified AHCI / RAID driver, and no need at all to install a RAID driver unless you are using a RAID array. That should mean that you ONLY install a RAID driver and set the port mode to RAID if you really ARE using a RAID array. Otherwise, set it to AHCI.

As bizarre as this sounds I was forced to revert to ACHI mode after updating my BIOS and the HDTach benchmarks indicated that my SSD had picked up 25 MiB/s using AHCI mode as opposed to RAID mode. The HDTach benchmark
also was pretty much a flatline at 475 MiB/s, but in RAID mode it had several
valleys that dipped to 450 MiB/s.
I don't have any RAID devices, I only used RAID mode because I wanted to use Intel's rapid storage technology mode (where you can use an SSD to cache HDD's), that didn't work out for me because I'm dual booting w/Windows XP Pro.
I'm actually thinking of adding a second SSD as another Readyboost device.

You threw in an interesting item, there. You're dual booting with Win XP Pro. Now, Win XP in all versions did NOT have any driver "built in" for AHCI devices, so it does not know how to use any storage device (HDD or SSD) acting (according to BIOS settings) as an AHCI unit. Thus it cannot BOOT from an AHCI device unless you install the necessary device driver for it at the time of the first installation of Win XP. (Using an AHCI device that you do NOT boot from in XP is easy - you just have to install the driver into Win XP after it is running.) So, did you actually do that installation in the original Win XP system on your HDD? Or, did you set the BIOS so that the HDD you boot XP from is operating in IDE Emulation mode?

On some BIOS's you can set the SATA Port Mode separately for each such port. But on some, whatever choice you make applies to ALL of the SATA devices on the main SATA port group. If yours operates in the latter manner, then this is what I expect will happen: Your system will boot just fine with the SATA ports set to AHCI mode when you boot into Win 7 or 8. But if you choose to boot into Win XP Pro from the other drive, I expect it will fail because it cannot find a way to deal with that HDD as an AHCI device. It probably would work if you changed the BIOS setting to IDE Emulation for the SATA ports, but then I'm not sure it could use all your storage devices when some of them have been written in AHCI mode. (You might solve this latter "glitch" simply by installing in XP while it is running the AHCI driver for Win XP on your hardware.) And of course, you would not want to leave your system set up that way and NOT use AHCI most of the time.

So, how does your system behave when you set it to use AHCI mode and then try to boot into Win XP Pro?
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