Samsung 840 Pro SSD & Gigabyte P55A-UD4P Slow?

I just installed a new 256g 840 Pro SSD. I have it installed on the 6g SATA port and have AHCI turned on. I did see some post that said some gigabyte boards would not get full speed on the 6g SATA ports.

The processor is an I5-750 at 2.67Ghz and has 12g of ram.

I ran samsungs performance check and I'm only getting 404MB/s.
I thought this drive was capable of over 500.

Any ideas? I'm going to try removing norton internet security and retest but I don't know what else to check.

The 6 Gbit ports are not native Intel ports. I did not look up that specific board, but most likely it has a controller (JMicron or Marvell) with a single PCIe lane (those with two or four lanes only became popular during the last few years) and is limited to 500 MB/s minus the PCIe overhead (which could result in 400-450 MB/s).

Since sequential throughput is irrelevant for most use cases anyway, I would connect the SSD to the 3 Gbit Intel ports. There they achieve the best possible performance. Low queue depth IOPS is what you should be looking at unless you copy large files between two SSD all day.

Even if they are not from the Intel chipset, you'll still get better performance overall from the Marvell SATA3 controller, so I'd leave it there. You can, however, run your benchmark on both controllers to verify that yourself.

I ran samsungs performance check and I'm only getting 404MB/s.

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Don't worry at all about that. You will almost never get 400+MB /s in real world applications anyways (since they do not read GB sized files sequentially or use high queue depths). What is more important for most applications is 4K low queue depth.