The annoying thing I have found with my 2013 New Mac Pro is that the USB ports seem weaker than on my old 2011 iMac. If I plug in one too many devices (even PSU powered ones) it can unmount or disconnect other devices. I only have a handful connected, one PSU (not bus) powered USB 3.0 drive and frequently have to disconnect it just to get my scanner or printer to be detected. If I plug in a WD passport then forget about other devices working properly.