The "external 5V" was not officially endorsed by Arduino.
The online docs mention that it's a valid power supply option, assuming it's properly regulated.
I think the problem is that the Mac turns off the port when It meets the external 5V.
You'd need to use a USB cable with the 5V wire cut off (only GND, D+, and D-) so that there are no conflicts.
Yeah I was thinking the same thing too - Arduino support got back to me and they didnt outright say its not feasible. I'll update the thread as info comes in
Or turns it of because there's no current draw.
This wouldn't be the case because if you power the board via Vin, Vbus is disconnected (so there is no consumption) and the port works the same.
Arduino team validated that I should be able to use the 5V pin to supply power so I think it has to do with the Mac port itself. I’ll try to get a hold of a data only usb-c cable but was having a hard time finding something online (power only seems to by much more popular). I have some dongles for data-only USB via USB-A so will probably test with that anyways but if anyone has links to USB-C data-only cables and/or USB-C power blocker adapters (DIY or commercial) to buy that’d be great. Rather try to buy something before exploring making my own
Yes, it can be powered easily via the 5V pin, but there's no provision for using the serial port (as can be seen from the schematic).
I think it'll be easier to cut a cable than to get one without the 5V pin (The common thing is the opposite, only charging without the D+ and D- pins).
Power blocked usb cable solved the problem. Hard to find USB-C data-only cable on the market so I used USB-A to USB-C cable with a power blocker adapter on the USB-A side. Thanks for the help everyone.