See my last post in this thread on page 5 for why I consider this issue solved.
I have a 4-port x4 PCIe SATA card inserted in the PCIe x16 slot of my mini-ITX MB. Preclears on any drive attached to this controller are reading at about 2.6 MB/s.
The MB is a Biostar TH61-ITX
On the MB SATA ports, the pre-read runs at over 100 MB/s
The PCIe SATA controller is based on the Marvel 88SX7042 chipset. I have tried both the SYBA SY-PEX40048 card and the Rosewill RC-218. They are ab