@mr-clay said in A fatal error occurred: MD5 of file does not match data in flash!: sorry for resurrecting this post from the dead but where exactly do you run the esptool --port <YOUR PORT> write_flash_status --non-volatile 0 command? You run this in a terminal or command line program. Replace YOUR PORT with the name of the port that appears in arduino. On windows this may be COM followed by a number on OSX this is often somelike USBslab.