You might need to update the firmware on the Prolific chipset in the enclosure. This website has the most recent firmware and ROMWriter for the B and C chipsets, which are the only ones that can be updated.
Also, you might want to look through this thread:
This has a lot of good info on updating the Plumax enclosures. You will mainly see info on the USB2.0/Firewire case, which I have, but it should help.
When you update the Plumax firmware, use ROMWriter 2.0.4. Then, once you reboot the drive, use version 2.2.0 to verify the update.