I’ve tried this on a Pi3B+ and a Pi4B and all it does is repeat error messages like the below:
/home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: 35: /home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: :00000001FF: not found
/home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: 32: /home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: :047FF000FF270994CA: not found
/home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: 33: /home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: :027FFE00040479: not found
/home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: 34: /home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: :0400000300007E007B: not found
/home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: 35: /home/pi/optiboot_atmega328_2012_with_1s_watchdog.hex: :00000001FF: not found
The files are there, they all have the 777 permissions, it just refuses to work.