Adjusted error message to include a possible path to resolving the issue

source/usbBrickIO.m

@@ -132,7 +132,11 @@ classdef usbBrickIO < BrickIO
             catch ME
                 if ~isempty(strfind(ME.identifier, 'CommError'))
                     % Throw a clean CommError to avoid confusion in upper layers
-                    msg = 'Failed to open connection to Brick via USB.';
+                    msg = ['Failed to open connection to Brick via USB. The most ' ...
+                            'likely cause is another EV3 object that is already ' ...
+                            'connected to the physical brick. Call ' ... 
+                            '"killAllConnections" using the EV3 object to disconnect' ...
+                            ' all other existing EV3 objects.'];
                     id = [ID(), ':', 'CommError'];
                     throw(MException(id, msg));
                 else