If more than one USB port, try another.
Try a different USB port and see if that helps. Also try using a different USB cable - some custom cables disable the power connection which may prevent the devices from handshaking correctly.
If that doesn't work...
The X-1 shouldn't need a USB driver but the older variants apparently did.
I don't suppose the MacBook Pro had drivers installed previously, did it?
So going back and forth with the Peachtree Audio guys (very helpful BTW) they determined there was a current leak across the USB ports between the Mini and the Grand. If I used my Apple Cinema monitor, which has a grounded plug as well as needing one of the USB ports to function, the problem went away. It also was able to be resolved by running a 3.5mm to RCA cable between the Mini and the Grand. So it comes down to a grounding issue. They offered to do a simple mod for free to resolve it permanently. Or I could just keep the 3.5mm to RCA in place....but that ties up an input on the Grand.
Haven't decided what to do yet. But good to find out there are a couple of possible solutions.