Sorry, I missed the above post for 2 weeks.
1. SYBA 2 port USB3 PCIe expansion card with NEC UPD 720200 chip
2. Seagate GoFlex Ultraportable 1.5TB external HDD - bare drive without any adapter/cable
3. GoFlex USB3 adapter/cable (USB3)
4. GoFlex USB2 adapter/cable (USB2)
1. When the HDD is coupled with the USB3 adapter/cable or USB2 adapter/cable and connected to the USB2 port,the transfer speed remains the same - 26.4MB/sec (screenshots 3 and 4)
2. When the HDD is coupled with USB2 adapter/cable and connected to the USB3 port, there is almost a 50% increase in speed - 39MB/second (screenshot 2)
3. When the HDD is coupled with the USB3 adapter/cable and connected to USB3 port, there is almost 330% increase in speed compared to the USB2 port connection - 87.1MB/sec. (screenshot 1)
This gives a general idea as to what one can expect with these combinations. Absolute values can be different for different brand HDDs.
(Keeping all conditions same, I have noted the transfer speeds in each instance after 50% of the file is copied - in this case 9.2 GB)
@ ian986, your screenshot seems to indicate that the WD passport is only performing as a USB2 device. Is it a USB3 device and did it come with a USB3 cable?