SO here is the situation: First this was my old PC, it had a 2x 1TB RAID 0 and a Corsair Force 3 SSD in it. This were the old speeds, measured by HDTune Pro.

2x 1TB RAID 0: HDD-old

Corsair Force 3 SSD SSD-old

Then my dad got my PC and we had several issues, in the end turned out both RAID and SSD controller were malfunctioning causing BlueScreens on 100% load. Removed the RAID 0, but leaving the HDD's intact and bought an Samsung 840 EVO 120GB, though the Corsair SSD is still in the system, just not as sytem disk anymore.

1TB HDD (one of them): HDD-new

Corsair SSD: SSD-corsair-new

Samsung SSD: enter image description here

We did not assemble the PC ourselves, so answering some technical questions might be more difficult, though we will do our best.

First thing we noticed is that the Samsung 840 EVO is no where reaching it's advertised speed, even an Samsung 840 250GB (non-EVO) is reaching 350 MB/s in my own PC.

Then we noticed that both SSD's are capped at 120 MB/s exactly, not sure if this is being caused by HDTune Pro, but very unlikely. And even worse, the Corsair Forza 3 was running faster before the system got reassembled.

Does anyone have any clue what is going on?


The potential causes can vary widely so please consider the following:

  • Your speeds are actually correct for both HDD and SSD if your motherboard is using SATA1 ports
  • IDE mode vs AHCI will also cause a considerable speed difference even on the same exact SATA version for both HDD and more noticeably on SSD
  • SSD companies have been known to benchmark using default settings with ATTO Disk Benchmark so that is what I use as well
  • If an SSD is more than about 50% full then benchmarks will suffer
  • If an SSD is malfunctioning/dying then benchmarks will suffer
  • Faulty/outdated SSD firmware can cause issues so update to latest firmware.
  • The SATA drivers for your motherboard might be faulty or too old to handle SSDs properly so make sure to update those
  • TRIM is hugely important for older SSDs whose controllers have a poor garbage collection routine
  • Vista does not support TRIM so hopefully your SSD can clean itself up
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  • It seems most likely that the SSD's are connected through SATA1 cables. Possibly nowadays motherboards do not have SATA1 cables anymore and the technician thought he could use any cable? Atleast a lead that's worth checking, together with IDE vs AHCI. – skiwi Oct 29 '13 at 16:10
  • I would like to correct your terminology a bit so that you don't go on a wild goose chase. It has been proven that a SATA cable from 2002 can candle the SATA 3 speed spec granted that the correct materials were used. maximumpc.com/article/features/… In my post I was referring to the actual SATA ports on the motherboard which are yellow in this picture img43.imageshack.us/img43/4074/sataports.jpg So just to re-iterate you need to investigate the ports and not the cable. – MonkeyZeus Oct 29 '13 at 16:21
  • Ok thanks will check on the actual ports then, I see. – skiwi Oct 29 '13 at 16:22
  • No problem. If you are having trouble getting started then first you need to figure out your motherboard brand and model number which can be seen by installing CPUz and checking the Mainboard tab buckjones.net/pages/chipsetdrivers/cpuz.gif Next you will need to search for the motherboard in google and find the manufacturers website and see what they listed for the SATA ports. You should also be able to get a PDF manual for the motherboard and check in there for the SATA version. Good luck! – MonkeyZeus Oct 29 '13 at 16:27
  • 1
    AnandTech's Smasung 840 EVO Review shows how most current controllers hold up under sustained writes, and his Corsair Force LS Review shows how garbage collection starts working over the course of an HD Tach run. Tom's Hardware Link shows how the Marvell controller in the m4/M5S and the Samsung 830(!) maintain performance. – afrazier Oct 29 '13 at 18:40

Speeds also depend on how many drives are "hooked-up". For example, I have 8 connections, 6 are labeled as 6MB/s. However, if you read the manuals, you will only get "sustained potential of 6MB/s", if you only have 4 units, and all 4 are on the first four 6MB/s connections. (Absolutely NO other drives can be hooked-up to any other port.)

If you hook just ONE drive up to any of the other 6MB/s ports, all ports instantly drop to 3MB/s max. Though, you can hook-up ONE more, to the other 6MB/s port, and they will also still be 6MB/s. Yet, if I have just TWO drives, they can be hooked-up to any of the 6MB/s ports, and run full speed. Adding the third drive must be done with use of the four first positioned ports, or the same as above, with 4+1 drives, will happen. (Essentially, there are 12MB/s per group, in the four ports at the front.)

Here is where it gets buggy...

Add one drive to ANY of the non-6MB/s connections, and the 6 ports turn into 2MB/s. The driver will then "burst", +2MB/s across any of the 6 6MB/s ports, but the four front ports will get an extra +2MB/s, if it is available.

To add insult to injury...

Adding an M.2 into slot-1 will reduce the top-row of 6MB/s into only 2MB/s slots. Adding an M.2 into slot-2 will reduce three, out of four on the bottom row, into 2MB/s slots, but the first one will remain as a 3MB/s slot, with that being the only one which gets +2MB/s bursts, as long as none of the M.2 cards are using ANY transfer bandwidth.

Yes, it is a big mess.

The same happens for PCIe slots. You may have 3 16x slots. But they are not all 16x, if populated. If only using one card, it will be 16x in any of the three slots. But, add one card in any slot, of any speed, and that card now only runs at 8x, and the other two will only run at 4x each, in many instances. (Eg, you only have 16x total lanes for data. Most controllers don't "float lanes". They have fixed reservations for bandwidth by locking-out lanes for other cards, when inserted. Sometimes/Most-times, RAM and DRIVES and INTERNAL VIDEO CHIPS and INTERNAL SOUND CHIPS and INTERNAL NETWORK CARDS, will also use those lanes, unless you disable them. Though disabling them may not actually free-up any lanes, it will reduce the interruptions from any "floating lanes", if some of those lanes are floating, in the 8x and 16x connections.)

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