The R-pi has a notoriously low amperage output on the (shared with the onboard ethernet) USB hub. Try plugging it into a proper (500ma - 2.1a) usb AC adapter (like the one that came with your smart phone) and see if you still see that behavior.
No prob Darren. To clarify, the device was visible to Virtualbox, and gave the option to pass it through to a VM. It wasn't visible to OSX's network preferences screen (so no option to set a static). I was however able to download the driver from realtek's site and get it working. This might inhibit a pentest against an el capitan victim, but for my own testing it works just dandy!