I was experimenting with external power supplies and noticed that the only way the Arduino can communicate with an externally powered sensor (through SPI) is when the GND pin under power AND the GND pin under AREF on the UNO is connected to the external ground?
From what I understand, both these pins are connected to the boards mutual ground (i.e. they are already internally connected)?
In the attached image both the orange and yellow wires were required and would not work individually.
I feel that either this shouldn't be the case or I am simply missing something and would appreciate an explanation.