Will a 6Gb/s HDD work on a 3Gb/s slot?

Indeed. There are only a few that can go faster then 1.5 SATA.

Unless it's a raid setup.

Best answer selected by shwetankjuneja.

RAID setup shouldn't change anything. Each SATA 1.5 channel would support 150MBs. If you used two disks that could do 90MBs avg transfer, then because its going through two channels you'll get the full 180MB. Even on SATA 1.5.

^+1.

The only time that SATA would slow down a RAID set is if you're using a hardware RAID controller that connects to the system using SATA, such that every I/O request between Windows and the RAID controller has to go through the one SATA connection. I don't know of any such RAID controllers. The internal ones connect via PCIe, the external ones usually connect via the network.

Sminlal, we might have had something similar pop up recently. We are waiting to hear back from the OP. He bought a PCIe 1x controller for his SSD and can't figure out why his read speeds are only ~200MBps. If he has this card plugged into a PCIe 1.0 port which tops out at 250MBps, then his 200MBps reading makes sense. He'd need to use a PCIe 2.0 which can move 500MBps if he wants his SSD to move faster. Not quite what you mentioned, but an example of how X connects to the system and you have a slow down.

Yes, I understand what you're saying. My point was that it's not SATA that's the bottleneck in these kinds of situations.