I may have fried something on my Uno. I have 2 Unos. One, I can flash just fine from arduino. The other one, also shows up as ttyACM0, but flashing gives me the stk500 not responding error.
If I use the ICSP header, I can reflash the Uno and then it works fine. I tried to reflash the 16U2 (my Uno r3 is the one that won't flash via USB/Serial), the procedure with dfu works fine, ttyACM0 goes away, then comes back, everything looks good. And then I plug it back into the arduino IDE, and it won't flash. If I plug my other Uno without changing anything, again that one flashes fine.
What are possible failure scenario there?