Suppose you wish to use an external USB Bluetooth interface instead of the built-in one. Reasons include using a newer Bluetooth version on an older Mac (keyword Yosemite Continuity feature) or putting the external antenna more near to your Bluetooth devices. First of all buy an adapter (Google helps you finding a compatible one), then

  1. Disconnect (unpair) all Bluetooth devices which are paired with the internal Bluetooth interface.
  2. Connect the external Bluetooth interface to a free USB port.
  3. Type the following command from an admin account on your Mac:
    sudo nvram bluetoothHostControllerSwitchBehavior="always"
  4. Reboot your Mac.
  5. Connect (pair) all Bluetooth devices using the Bluetooth control panel of System Preferences.

The external adapter will be used for all Bluetooth related operations until you say

sudo nvram -d bluetoothHostControllerSwitchBehavior

and reboot again. (Don’t forget the unpairing step before the reboot.) An Option-click onto the Bluetooth icon in the menu bar (activate this in the Bluetooth control panel of System Preferences) reveals the hardware address of the Bluetooth interface currently in use. Use System Profiler to verify that this is the correct one: In the USB section, both the internal and external interface are listed.