"USB Needs more power." - Mac Pro with 7 Keyboards? I am in the process of setting up a MacPro whose sole purpose is to run VMs on each of the six possible screens each with it's own keyboard and mouse.
So what I have is six DVI monitors with six dual-link display-port to DVI/USB adapters and an Anker 10 port USB hub/charger to power them all.  The VMs are to be each assigned their own keyboard through virtualbox USB filters.

Originally the monitors were having some trouble coming up all at once (and in order) with a Macally 7 port hub so I'd thought the issue was the amount of power the hub was outputting.  Additionally none of the keyboards except for the one attached directly to the MacPro itself were being recognized so more power was figured necessary.

However even with the more powerful Anker USB Hub the system will not work with the extra keyboard and mouse attached through the hub with a single monitor running.  The "USB Needs more power." message still comes up. These all happened to be old extra Apple keyboards.  I've also tried with a couple of non-Apple specific keyboards and it appears that they only work when hooked up directly to the MacPro even though the USB power message does not come up. Next I'll be trying separate hubs for the display port hookups and the keyboards and mice to see if it is some sort of weird dual-link display-port to dvi issue.

Will MacPros only recognize a single Apple Keyboard? Only keyboards hooked up directly? Is this by chance a software issue I can disable or work around with extra software?
This kind of applies to all hardware, not just apple computers. I did a search for Anker USB Hub. None of the results showed a powered USB Hub. They all relied on the USB Bus for power. 

The average USB port has about 500 mA available. (900 mA in USB3.0) USB Keyboard uses between 50 mA and 100 mA. The USB hub itself also needs power, more if it's got all those pretty LED's on it. This puts the load between 400 mA (idle?) and 800 mA (all in use?)

So, it seems like your using more power than a single USB port can provide. You may just need to upgrade to a hub that supplies it's own power to devices. Which would require a hub with an AC adaptor.