K-H, I just updated the firmware on both the A7RII and the A9 on my trashcan. It is still on Yosemite as I usually run one version behind - partly from laziness and partly from caution I guess
I tried it initially from the USB hub and it wouldn't recognize the camera although the little yellow light did come on when the cable was connected.
So I plugged the cable into the trashcan directly into one of the USB ports and it worked. I used the original Sony cable that comes with the camera as some people have found other cables do not work (I see nothing special about this one but played it safe).
what is annoying is the time it took. Over 15 minutes for the A7RII and over 11 minutes for the A9. At one point with the A9 (I started with it first), I thought it was stuck and I was in trouble since nothing happened for several minutes the progress bar seemed to be stationary. Then it picked up.
With the A7RII, even after clicking on 'Finish' in the updater software, ejecting the camera and turning it off, for over a minute the screen remained on with the message 'Please wait, retrieving data'. I thought Oh boy, something must have messed up. But happy to report all is well and the camera now reports version 4.00
what a pain, but I do believe it will be worth it.