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If I add a SATA 6 Gbit/s PCI Express card to my motherboard, will I get similar speed to native SATA ports built into the motherboard? I ask because I have a SATA 6 Gbit/s SSD and my motherboard supports only SATA 3 Gbit/s.

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It depends on whether the PCIe SATA adapter uses the 1x or 4x bandwidth capability. The PCIe 2.0 specification defines 1x slot bandwidth as 500 MB/s, which is much higher than SATA-II. If you were to purchase such a card (like this one), it has a data transfer rate of 4.69 Gbps.

SATA uses 8b/10b encoding, which translates to a raw maximum speed of 469 MB/s. Compared to SATA-II, which caps out at 300 MB/s, you have 1.5x the bandwidth. Not full SATA-III, but a lot better than your motherboard's native SATA-II ports.

Note that this is limited just by the PCIe 1x port. Using a PCIe SATA adapter that can utilize a 4x slot instead, you can remove this limitation and are now at full SATA-III speed. (And such adapters do exist.)

Finally, note that if you have an older motherboard (i.e. PCIe version 1.0), your bandwidth will be exactly half of what I've listed above.

Whether or not you actually need the increased bandwidth is another issue altogether, and I would recommend that you see my answer to "Is it worth to get a SATA 3 controller to max the [drive] out?".

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  • Note that PICe 1.0 provides 250 MB/s and SATA-II provides 300 MB/s, so if happen to be running PCIe 1.0 then you're definitely better off sticking with the onboard controller. – nedned Oct 28 '14 at 14:38
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Unlikely. None of the third party SATA 6Gbps controllers I have seen have been able to match the performance of native SATA 6Gbps from AMD and Intel when connected to a SATA 6Gbps SSD.

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SATA3 cards will only benefit from a SATA3 mobo, if SATA3 runs at 6GB/s and SATA2 runs at 3GB/s as you may know. The SSD you have won't be running at it's rated speed due to this limitation, you can either buy a new motherboard that supports SATA3 or stay with SATA2 purchases. So to answer your question don't add a PCI Express SATA3 card if there's an equivalent SATA2 card available, you can save yourself that extra few $ as well.

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    This answer is completely wrong. PCIe 2.0 slot bandwidth is higher then the SATA-II bandwidth. Products like this one, for example, will provide roughly 1.5 times the bandwidth of your SATA-II ports. It may not be double as in the case of SATA-III, but it sure is better then what you have now. Find a PCIe adapter that uses a 4x slot, and you're now at full SATA-III bandwidth. – Breakthrough Jul 12 '11 at 13:14
  • Sorry I was mistaken on the answer as I thought he was asking if it would match the speed of a SATA3 controller. – Sandeep Bansal Jul 12 '11 at 14:18
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    @Sandeep Bansal, again not quite correct. These cards have an actual physical SATA-3 controller on them, and in most cases, they are the same ones on the motherboard itself (the one I put has a Marvell controller). They are even both accessed by the CPU through the southbridge - the difference lies in the interconnect/interface bandwidth, and that's it. – Breakthrough Jul 12 '11 at 14:29

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