did you ever figure this one out? I have an asus laptop with a 3400 series eSATA port and only 1 drive is recognized in a rosewill rsv-s8 (sil3762 port multiplier). granted i'm running linux, but if you have a solution that you found I would be grateful if you shared it to help figure this one out, thanks!
Yes I do have an answer, though it is not a solution. It turns out that the 3400-series chipset, no matter what board it is on, does not include the necessary instruction set to facilitiate port multipliers on the SATA ports. This was documented in the 3400-series chipset Technical Product Specifications document, and the limitation was intentional on Intel's part, probably due to limited memory addressing space for the storage subsystem. There is no BIOS or firmware that would fix this problem. I ended up having to install several SATA RAID PCIe HBAs that were compatible with my sil3726-based port multipliers in my server board for my storage drives, and used the on-board SATA ports strictly for my boot drive.
This is probably not the answer you wanted to hear, but I recommend looking for a PCI ExpressCard that has compatible eSATA ports for your hard drives, assuming your laptop has an ExpressCard slot available.