I am also a newbie but did you try disconnecting every single component and trying connecting the Arduino isolated.
Yes i tried that. I actually have the same issue with another arduino UNO unit. The problem remains even after unplugging everything. It's not just the setup that causes a malfunction, it's the arduino unit itself.
The possibility is there, try this. Start by removing everything connected to the arduino then try this:
Take these steps:
Power off the board completely (remove the USB cable).
Wait about 15 seconds.
Hold down the Reset button, keep it held down (or, run a jumper wire from the RESET pin to the GND pin).
Still holding down Reset, reconnect the USB cable.
Start uploading a sketch that does not have this problem (I use the Blink sketch).
I have seen this happen several times when I flood the serial output. An indication is the tx light is on.
Try this several times, it does not always work on the first pass.
Secret do not keep the console constantly busy printing. I put a 5 second delay in the setup so I can get control incase of a problem like this. Then remove all the motors from the 5V and power them from another power source, The 12 volt can be regulated to the 5V for the motors. Be sure to connect the grounds.
Good Luck & Have Fun!
Thanks for the info Gil! It doesn't seem to work however. I have a second Arduino unit with the same issue (i bought a second one as i didn't have the time to figure out what was happening). I have the second Arduino unit, but when connecting to the computer (Apple Macbook), it fails to recognize the device so I'm completely unable to upload anything whatsoever (that includes the blink sketch). I checked the Uno unit for possible burn marks that would evidence a current overload, but there is none to be seen...
This is what i tried so far:
I tried a different USB port, I tried putting a USB HUB in between, I tried another laptop, I tried a Windows PC (which, in fact, could recognize the Arduino but uploading took about 2 min), I tried updating drivers and FTDI, I tried another USB cable... Nothing seems to solve the problem. The only thing that really has worked so far is just getting a new Arduino unit. I'm really getting desperate because I can't keep on buying Arduino units :D.