I have tried using 4 different SATA3 cables that came with the Asus motherboard, 3 different Vizo SATA cables, 2 different Amphenol SATA3 cable that came with Intel 510 SSD, all had 6Gbps with one SSD configuration and as soon as I put 2 Intel 510 SSDs into the system, the drive randomly jump back and forth between SATA3 and SATA2. Fresh boot up will be SATA3 mode, but most of the time in SATA2 mode. Signal degradation does not explain why single SSD configuration is fine and problem only occurs when I have two SSDs configuration.
When in SATA3 mode I can get sequential reads faster than 450MB/s, in SATA2 mode it drops to like 280BM/s.
i7-2600 w/ Thermalright Archon
Asus P8P67 Pro
2 X Asus GTX460 DCU TOP 1GB
4 X 4GB Corsair Vengeance DDR3-1600 1.5v
Intel 510 SSD 120GB + 250GB
Hitachi 7K3000 HDD 1.5TB
Coolermaster Silent Pro Gold 800W
Please note that I may not be a pro system builder, but I have build over 20 PCs for myself and my friends over the years. I have ruled out most of the possibility of hardware conflict by cross checking/interchanging different components. I am forced to conclude it's a controller chip driver issue or SSD firmware issue.
Since Intel refuse to address the problem, I will temporary move one of the SSDs to upgrade one of my laptops until a proper fix is provided by Intel.
Just an update on the situation. Intel did send me a new 510 SSD 120GB with firmware PPG4 to replace the one I sent in for RMA. In 2 SSD configuration, the speed randomly drop from 6Gbps to 3Gbps.
So I moved one drive into my laptop. Now, I notcied, even in a single SSD environment, the speed randomly drop from 6Gbps to 3Gbps.
I hope they release a new firmware for the SSD or new version of Intel Rapid Storage Technology. Obviously, something is not right.
Update: After installing iRST 10.8, the SSD doesn't drop to 3Gbps any more.