Here are the details.
Everything worked fine for the past 2 months (in terms of uploading), until I disconnected the ethernet cable on my project (which connects 2 button boxes using SPI, which was also working just fine). Then the LEDs went crazy, and then I could not upload any longer, and the boxes no longer work.
No new software or updates have been done to the computer I am using for the Arduino. I am using the most up to date OSX.
I've checked everything listed on the troubleshooting page you mentioned:
Arduino software is up to date.
The Arduino board selection is Arduino Uno (the one I have).
The port is correct, and the same as before (ie the past 2 months): "/dev/cu.usbmodem411 (Arduino/Genuino Uno)"
The Serial Monitor opens (if the board is plugged into the usb port) just fine, as it did during the past 2 months.
The Arduino green On LED is lit when the Arduino is plugged in.
The yellow LED flashes as usual when I first plug in the Arduino. If I change the main Arduino chip to a new one I have (no bootloader installed), this light does not flash.
The upload error:
avrdude: stk500_recv(): programmer not responding
avrdude: stk500_getsync(): attempt 1 of 10: not in sync: resp=0x00
(It tries 10 times)
Problem uploading to board. See http...
The upload error with Verbose output:
avrdude: usbdev_open(): did not find any USB device "usb"
(And then the Arduino software stays at "uploading" until I try again.)
Menu Tools/Get Board Info yields:
I tried the loop back test failed: I can type into the serial monitor, but there is no echoing. Also, no Sx or Tx light up.