Summary

Flash:

I hope those frustrated with a board that won't flash will find the following useful.

What do you do if you want to flash a new firmware in to an Arduino-based printer but the blasted thing won't take and you've done the usual things like:
-Making sure you have the correct drivers installed
-Making sure you have your programming device / software configured properly
-Is the cable plugged in properly, did you try another one?
-Did you try flashing from another computer / OS version?
-If your board supports it did you try setting it for power over USB first?

...and yet the blasted thing still won't flash!

It seems this not an uncommon problem, especially for those of us with Trigorilla boards, and possibly other ATMega-based systems.

There's one last thing to try before you go buying another board - re-flash the boot-loader and then have another go at flashing your firmware.

When you flash a firmware to an Arduino-based system (usually over USB) the micro controller won't have a clue what to do with it unless it already has a small bit of code called a boot loader...
...now the idea is that the boot loader should only need to be installed once as it doesn't get overwritten when you upload an Arduino Sketch (printer firmware) to your device. But the boot loader turns out to be a bit prone to corruption, especially if you've flashed your device a few times.

Symptoms may be:
-Something happened to brick your board. ie memory contents scrambled to the point where nothing works and attempting to resurrect the board with a firmware re-flash just doesn't work
-A fully working machine that just refuses to take a new firmware

What is certainly worth trying is to 'go back to the beginning' and reinstall the boot loader. The trick is doing so cannot be done via the friendly USB port, it has to be done via the board's ISP header using a widget called an ISP programmer.
The good news is if you're happy to flash a new firmware to your printer then re-flashing the boot loader is really not harder - it's just a different process. I would strongly recommend Arduino's own guide for getting the job done:

https://www.arduino.cc/en/Tutorial/ArduinoISP

Important:
When you re-flash the boot loader via the ISP connection everything else will get wiped out so:
-Don't be surprised if your printer looks totally dead after
-Do be sure you have a backup of anything important
-Be sure you have a working printer firmware on hand

...once the boot loader is back in place you should be able to flash a firmware to your machine once more via the USB port.

DISCLAIMER
While doing the above really isn't that hard I would regard it as a final step to try before tossing your board in the bin and getting a new one.
-If the boot loader was the problem you'll be very happy you gave it a go
-If it doesn't work you're not worse off
-If you killed a working board to satisfy your curiosity you'e an idiot and only have yourself to blame.