do not use hub but do reboot between two successive installs
hub will provide power (even unpowered hub) for testing but not enough for stable operation, which must be negotiated.
eventually i stopped getting errors about pre-load etc from
sudo kextutil -tn /Library/Extensions/SiLabsUSBDriver.kext
avplay -f video4linux2 -channel 0 -i /dev/video0 & avplay -f alsa -i plughw:3,0 -nodisp
avconv -f video4linux2 -channel 0 -i /dev/video0 -f alsa -i plughw:3,0 -vcodec mpeg4 -vtag DX50 -b 1200k -r 25 -acodec libmp3lame -ar 44100 -ac 2 -ab 128k -vf yadif,scale=720:480 -y test.avi