I doubt ACHI had anything to do with the better performance. I've never seen ACHI actually increase hard drive performance...
When you see large drops like during the first test, it's usually a sign alot of times that there are background programs or processes running that are causing delays while running HDTune.
With not factoring in the large drop at the end of your first test, the minimum and maximum transfer rates are pretty much the same, along with Access Times, Burst Rates, and CPU Usages.
Try running the test again with ACHI off, maybe you'll see different results, more along the lines of what was seen in the second test...
When you see large drops like during the first test, it's usually a sign alot of times that there are background programs or processes running that are causing delays while running HDTune.
With not factoring in the large drop at the end of your first test, the minimum and maximum transfer rates are pretty much the same, along with Access Times, Burst Rates, and CPU Usages.
Try running the test again with ACHI off, maybe you'll see different results, more along the lines of what was seen in the second test...