I recently spent an hour of tinkering and struggling trying to upload the GRBL hex file to Arduino via the mac terminal. I tried a lot of different parameters and in the end realised that most of the tutorials are for the older Arduino and the new Arduino communicates by default at a different baud-rate (possibly linked to the great Arduino divide). I also found it necessary to add the -F flag as I kept getting the message, “Yikes! Invalid device signature.” I post my final terminal command here to speed up the process for others and for posterity/reference.
/Applications/Arduino.app/Contents/Java/hardware/tools/avr/bin/avrdude -C/Applications/Arduino.app/Contents/Java/hardware/tools/avr/etc/avrdude.conf -pm328p -cstk500v1 -P/dev/tty.usbmodem1421 -b115200 -D -Uflash:w:/Users/alcwynparker/Downloads/grbl_v0_7d_atmega328p_16mhz_9600.hex -F