Hi, I have problem with USB ports on the left side too, under OSX. I think, it is caused with low power.
This is my detection under OS Sierra on my M4800:
If I want use some small or pocket USB key (it doesn't matter on capacity), it works properly via all of the USB ports (1, 2, 3, 4 and eSata)
But, if I want use external HDD (fe. 2,5" HDD WD Elemetns 500GB), it works and shows as connected on USB 3.0 Bus only in one port on the right side (USB 3.0 speed) and also on eSata port (USB 2.0 speed). This HDD is connected via single cable.