I am encountering a problem with Android ADB debugging through USB.
Using Macbook Air 2013 and trying to connect Nexus 6 dev phone.
On the MAC Android Studio
If you are using VirtualBox and have a virtual machine opened, you are probably trying to use adb from your host while it is plugged on your virtual machine. The problem is that you cannot have your device plugged on both at the same time.
Solution : Uncheck your device on the bottom right panel if it is detected by your Virtual Machine.