Green LED on Pin 13 blinks once on boot

Is there any way to disable this, short of overwriting the bootloader?

Is there any way to disable this, short of overwriting the bootloader?

Well the only code running at boot is the bootloader so the only way in software would be to modifiy the bootloader code. You could just remove the LED from the board, it it’s real important to you I guess.

Do you have one that has pin 13 connected to the LED? If not then if you absolutely need the pin disabled because you have to use it and the blinking messes with something you may want a custom bootloader.

If you have an older arduino you can cut a trace joining the LED and pin.

If pin 13 causes problems with some finicky device then find a way around it by putting something non critical there and the finicky device somewhere else.

It's not the LED which is on the board which is the issue, it's an LED on my display which is the issue.

I guess modifying the bootloader is my only option.

What display?

it's an LED on my display which is the issue.

So how is this being powered. When the boot loader is running all the arduino pins are in input mode. This can cause some signals to float high and turn things on that shouldn't be on. A well placed pull down resistor normally solves these sorts of problems.

The cathode of the led is connected to pin 13 via a darlington array. So pin 13 goes high, and that pulls the cathode low.

The anode of the led is connected to pin 1, ie: TX.

So if I had to guess, I'd say the Arduino is attempting to transmit at the same time as it's blinking the green LED.

The anode of the led is connected to pin 1, ie: TX.

Yes that is part of the normal boot loader operations, so if you want it not to do this you will have to modify the boot loader. However, I don't know how you will do this and still have a boot loader that is operational.