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.
Best Answer
I am pretty sure this is an issue with macOS Sierra. I had a similar problem with a brand new powered hub (Dyconn PowerHub SuperSpeed 12 Port (10 Data/Charging + 2 Charging Only Ports). I assumed it was a defective hub and replaced it. However, the problem persists with the new hub. I have searched around and there doesn't seem to be systemic issues with that hub, and there are lots of issues with macOS Sierra and USB power management.
Currently I have a powered hard drive and 2 printers connected to the hub, so I don't think anything is drawing power. The devices seem to work - I can print and access the drive, but about every 30-60 secs a "USB Accessory Needs Power" window opens and closes very quickly.
I am running macOS Sierra 10.12.4 on a MacBook Pro 15 mid-2015. From what I have read elsewhere, macOS Sierra has issues with USB management.