Off brand Pi Pico VSYS PSA
Hello everyone, I haven't seen this issue mentioned before so I decided to make a quick write up.
I ordered a picoboot install kit from Amazon that included the off brand purple Pi Pico seen in the photo. After installation I noticed that the pico would not power on from the 5V source on the GameCube, only if it was powered externally by USB.
I followed the github link printed on the back for this particular board and found that the PCB layout is slightly different, where there should be a 100K pull-up from VSYS to 3V3_EN on the genuine board it is instead connected to VBUS. This means the pico will not power up even if there is 5V at VSYS as it is not generating the core 3V3 voltage.
The solution to this issue is to solder an external 100k resistor from VSYS to 3V3_EN as shown. Also, this issue can be avoided entirely by using a genuine Pi Pico which I would recommend.
Hopefully this saves someone the couple of hours it took me to diagnose.
*NOTE: I have not tested whether this causes issues with simultaneous USB and GameCube power, I think it will be ok but be aware.