Obviously it's related to one or more of: the card, the driver, the enclosure, or cabling. My bet is a combination of several factors: probably something to do with the card (maybe firmware related, maybe driver; note that the driver you're using is
4 years old), and something to do with the enclosure as well; in my experience, not every port multiplier (in the enclosure) works with every eSATA card, even though they're supposed to.
Tracking this sort of problem down almost always requires direct access to the hardware, which really means you're going to wind up solving the issue yourself. About the only real suggestion I can give is to determine (by disassembling the enclosure
if needed) what the first four (0-3 or 1-4) SATA connectors are for each of the two port multipliers in the enclosure, make sure all drives are inserted into one of the corresponding slots, and see what happens. This may solve the issue; if it does, the highest
two slots will never be usable with your combination of controller and enclosure. If the issue persists, move the drives connected to the "low" connecters to the "high" connectors and see what happens.
I'm not on the WHS team, I just post a lot. :)