A common cause of communication errors that you'd see with the GPIB-USB-HS (and not with the PCI-GPIB, with the same code) is the USB port is not providing enough power to the bus. If you try another USB slot or hub, do you see a difference? Do you see the same errors on another computer with the same USB-GPIB-HS?
What are the communication errors that you see?
Regards,
Missy S.
Project Engineer
RoviSys