I started searching for an add on card for my machine because I am blessed with a Marvell 9128 SATA III controller that really sucks. It looks like the majority of standalone SATA III controller cards are implemented on a PCI Express 2.0 x1 interface. Most with at least 2 ports. How the heck do they get full bandwidth out of one PCIe 2.0 lane? Unless I am crazy SATA III is 6Gbps and one PCIe 2.0 lane is 4Gbps. Am I missing something here? Why don't they put these on a x4 or x8 even?
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You are correct - an x1 controller will limit your bandwidth and slow down your drive. A good SATA 6.0gbps expansion card is going to be pretty expensive. Several of them will be x8 cards.