Hard drive(s) not appearing in Disk Management, definitely a driver issue

Problem: I have sata hard drives in a multi-bay external enclosure that do not show up at all in Windows Explorer OR Disk Management when connected using eSata. They only show up in device manager.

Specific Details: I'm using a Mediasonic HF2-SU2S2 Pro Box in eSata mode connected to a cheapo PCI-E x1 add-in card with Silicon Image SiI 3132 chipset. The OS is Windows XP SP3. My exact problem is that the hard drives will appear in device manger under the "disk drives" category, but no where else. They do not appear in Windows Explorer or Disk Management at all. These drives are formatted properly and have data on them. In fact, I have been using these same drives in the same enclosure in USB mode.

If I look at the driver properties for the hard drives in device manager, I see that windows has NOT associated disk.sys as a driver file. All my internal drives (and my drives on my other systems) ALL have disk.sys in the driver details. I'm pretty sure this is the cause of my problems, but I don't know how to make windows to use this driver file. I've tried the "update driver" option and telling Windows to search specifically in the C:\Windows\system32\drivers folder but it just returns "This hardware was not installed because the wizard cannot find the necessary software."

Is there some way I can FORCE disk.sys to be loaded with the drives?

Things I Know/Tried:
1. The PCIe card has been flashed to the latest non-raid BIOS and drivers updated.
2. I am absolutely positive the hard drives are formatted and functioning properly.
3. The SiI 3132 chipset on the PCIe controller card supports port multiplier and sata hot swap.
4. The enclosure/controller card combination works perfectly in another computer (Windows Vista SP2 32bit). Multiple drives detected and appear in Windows Explorer.
5. OS is Win XP SP3