Admant:
startup items I don't think, because the system doesn't even start, it stuck before the system boot, so it can be a bios thing, but how to verify? how can I access the bios on a mac?(for the sake of completion, it happens with the bare minimum example sketch uploaded, so it is not a code related problem)
A
Does the RX TX blink? maybe the PC is asking the device something and waiting, it is weird but in an old pc I remember having the legacy USB (mouse/keyboard/pendrive) support enabled halted the start when I had a SD card reader plugged