Thanks Koepel. My SD reader has a built in level shifter, but not the MPU. Now I’m going to go back and look at all of my modules/interfaces.
I’m using a serial TTY radio and GPS. Both are 3.3V tx/rx. Both are working fine, but I suspect the Mega is pushing 5V. Can anyone confirm the standard voltage on Serial1 and Serial2 for the Mega? I see that the Due uses 3.3v and the Uno 5V, but I can’t seem to confirm the TTY voltage for the Mega.