It appears, like the Arduino UNO, there is an LED connected to Pin 13 of the Arduino Pro Mini. But so far I have been unable to use it as an input. Pin 13 of the UNO works fine as an input, but not on the Pro Mini. What or why the difference?
On the Uno, the LED is buffered with an opamp, so that even a high-impedance input signal which wouldn't be strong enough to drive the LED on it's own can be used). On the pro mini, to save space and cost, the LED is driven straight off the pin, so if that pin is used as an input, it can only be used if the signal is strong enough to drive the LED. You can remove the LED if you need to.
I see the difference now. I set pin 13 with the internal pullup and the switch is a dead short to ground. I expected this would be the required way to use the pin with the LED, but not?