Hi James,
Even though the VCC_3V3 (Red), D0 (Yellow), D1 (White) pins output the same 3.3V, the maximum power draw allowed for the pins are not the same. Here is a schematic of the Digital/Analog Grove Port to start with.
All the I/O pins from the ATMSAMD51, except the 3.3V and the 5V pins, output 3.3V with a maximum current draw of 15mA. You can see it from the ATSAMD51 datasheet in the below section:
Therefore each D0 and D1 pins can only draw 15mA of current.
However, the VCC_3V3 pin comes from the low-dropout (LDO) voltage regulator circuit as follows.
Therefore, the VCC_3V3 pin can draw a maximum current of 2A.
I assume the fan you are using draws much higher current than the D0 and D1 pins can handle which is 15mA. And it is possible to work on the VCC_3V3 pin because your fan might need a current less than 2A. You can look into the current specifications of the fan to confirm this.
Alternatively, if you want to use a fan, you should use a fan which draws less than 15mA of current. Also, you should carefully check about the current draw from devices before connecting, because a higher current draw from a pin more than it’s rated current can permanently damage that pin.
Hope it’s clear.
Best Regards,
Lakshantha