Does using one USB port reduce performance on others? This was a difficult topic to search for on the Web, so I'll try asking here:

On a MacBook Air (13-inch, Mid 2013) with 2 x USB 3.0 ports, if I transfer files to an external drive using one port, does this in any way "eat up" the performance of the other USB port? Or does each port have its completely own bus / capacity? 

A relevant side question is how the same applies to other multi-port setups such as:


the USB ports on Cinema Displays
the USB ports on Thunderbolt Displays
the Thunderbolt ports on newer MacBooks
I have often faced this interesting question. Apple has always tried to make its operations transparent - and although this is fabulous, sometimes this approach obscures information we want. 

To discover how many busses your machine is running (as you rightly distinguish from mere ports) go to system profiler, and select USB on the left. 

USB is listed amongst all the other protocols within your machine. 

You will then be presented with a device tree which (usually) gives a pretty clear idea of what’s going on with the bussing. 

If you plug things in to the ports you should see the devices come up. 

To get to system profiler, select “about this mac” from the Apple Menu and the click "system report" in the window that comes up. Another way us to call up spotlight by pressing command and space bar together, type system profiler and launch it directly from there. 

When you are looking at the usb device tree you may be surprised to learn that the usb bus is also in use internally - for example on some older macs, Bluetooth was running off the buss internally. 

Hope this helps.