Ok, I believe I now have the correct command. Now it says the programmer is not responding:
Roberts-MacBook-Pro:~ robertwinter$ avrdude -p t45 -P /dev/cu.usbmodemfa131 -c arduino -U flash:w:firefly.hex
avrdude: stk500_recv(): programmer is not responding
avrdude done. Thank you.
Then if I immediately try the same command (I copied the command so I could easily paste it and try again) I get a different error:
Roberts-MacBook-Pro:~ robertwinter$ avrdude -p t45 -P /dev/cu.usbmodemfa131 -c arduino -U flash:w:firefly.hex
avrdude: stk500_getsync(): not in sync: resp=0xe0
avrdude done. Thank you.
At this point I think there is a problem with the way the Arduino is set up but I could be wrong. I should also note that the L LED is solid, and the RX and TX only blink once, followed by a long pause before terminal gives me the error message "programmer is not responding." When I get the "not in sync" error the L LED is solid and the RX and TX LED's blink and immediately I receive the error message ("not in sync").
Is there something wrong with my Arduino?