This problem happens on both a 2011 Mac Air (Sierra) and a 2013 Macbook Pro Retina (El Capitan).
For a couple years, I have had a powered USB 3.0 hub with a few devices plugged in, including a KVM, and an apple usb 2.0 ethernet adapter. I move it between the two laptops above, and everything was great.
Recently, I've been having trouble. The Mac puts up a notification "USB Accessory Needs Power / Connect "Apple USB Ethernet Adapter" to a USB port on this Mac." If I do that, it works, but mysteriously, if I plug an old, non-powered USB 2.0 hub into the USB 3.0 hub, and plug the ethernet device into that, that also works.
Every once in a while, the keyboard connected to the KVM stops working, too.
What's wrong here? Is the hub defective? The ethernet device? Why does the second hub help? Since it happens to two laptops, it doesn't seem likely that it's a laptop or driver issue.