OK, I'm having this message each time I attempt to upload a new sketch, even the simplest one.
I'm using the original Arduino software, on Mac OSX 10.6.8 with an Arduino UNO R3
I searched on the net and tried all kind of solutions, even to try to reset the board just 2 seconds before uploading.
The board worked perfectly until yesterday.
The board is connected to a direct USB port, not to a hub.
The board is giving the same message from a Microsoft Vista portable computer.
The selected board type is correct.
No circuit is connected to the board. It is simply the Arduino UNO, powered directly from the USB.
The serial port, maybe, is the problem, because I cant see the /dev/tty.usbmodem621 that I used since yesterday ...
But it also disappeared from the Vista portable device.
Any suggestion?