I have a VERY similar problem to 'rubbed'.
I too have a Samsung 256 GB 840 Pro. At first when I tried activating ACHI it would not boot but would boot fine when set to 'IDE". Someone said they thought IDE was only a name or tag. In my system BIOS you set the drives to either IDE or ACHI. I was eventually able to activate the ACHI by going into the registry and editing a couple of keys and then it would boot to ACHI. I did benchmarks with both Samsung Magician and CrystalDisk Mark and I did see an improvement in the benchmarks for before ACHI and after.
However, I have a laptop that I put a Samsung 840 EVO in only 3 weeks before. Both my laptop and my main tower PC are only SATA 2 and both have ACHI activated but the laptop SSD is "FASTER" than my tower PC?
The laptop (LT) is an ASUS with Intel dual core 2.5 MHz with 4 GB of RAM........... The tower (T) mobo is also an ASUS with a Intel i5 2.85 MHz and 16 GB of RAM
(LT) 840 EVO SSD speeds are SEQ 532 & 357 and RAND 53729 & 37195... (T) 840 Pro SSD speeds are SEQ 274 & 240 and RAND 49752 & 38015
Or laptop Seq. 532 vs. tower 274; and LT 357 vs. T 240 = Laptop average 70% faster
An (T) Random 53729 vs. 49752 & 37195 vs. 38015 = 6% faster
So there is a huge difference in the sequential read/write where the laptop is 70% fast but in the random read/write only 6% faster! BUT THAT IS an 840 EVO in a LAPTOP average 40% faster than a 840 PRO in a tower PC!
SOMETHING is obviously wrong??
ANY HELP will be greatly appreciated.
TIPS for others (I have learned): First when setting up an SSD disconnect all other drives UNTIL you have the SSD setup and booting with it set to ACHI. Do NOTHING else until you do that.
Also make sure it is recognized as DISK#0 and ACHI, before you install your OS system. I had an older HDD 1 TB with a lot of data on it, partitioned into 4 drives. When I first booted up it had the partitions ID'd as C: D: E: and F:, which I had to change so I could have the SSD boot drive as the C: Drive.
So remember SSD first by itself, until BIOS is correct identifying the SSD as you want it to.
*** *** *** *** *** *** *** UPDATE *** [Feb. 4, 2014] *** *** *** *** *** *** ***
Samsung came out with an update in the software for the Pro SSD, the "Samsung Magician Pro". It now has a feature called "Rapid Mode" and it really speeded up my Samsung 256 GB 840 to the speeds higher than indicated that they should be? But they do vary a great deal, so I am not to sure about these tests by the Samsung software. However, they are 3-4 times faster than before, with the same benchmark software?
This is the CRAZY SPEEDS I get now (first today):
Seq.Read 1231 (Up to 540) Seq. Write 651 (Up to 520)
Random Read 81380 (up to 100100) Random Write 68949 (Up to 90000)
And three weeks ago
Seq.Read 983 (Up to 540) Seq. Write 835 (Up to 520)
Random Read 72940 (up to 100100) Random Write 87760 (Up to 90000)
These speeds are improved over the old speeds by 50% or 1.5 times fast to 100% or double the speed.
Old was like:
Seq.Read 343 (Up to 540) Seq. Write 436 (Up to 520)
Random Read 49752 (up to 100100) Random Write 87760 (Up to 90000)
The statement "Up to ---" is Samsung's expected speeds for this drive.
Previously