I am having the same problem with my mega 2560
Arduino: 1.6.4 (Windows 7), Board: "Arduino Mega or Mega 2560, ATmega2560 (Mega 2560)"
Sketch uses 6,916 bytes (2%) of program storage space. Maximum is 253,952 bytes.
Global variables use 468 bytes (5%) of dynamic memory, leaving 7,724 bytes for local variables. Maximum is 8,192 bytes.
avrdude: ser_open(): can't open device "\.\COM8": Access is denied. avrdude: ser_drain(): read error: The handle is invalid.
Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.
This report would have more information with "Show verbose output during compilation"
enabled in File > Preferences.
or it remains uploading forever...
I checked the firmware info and I have rev 0001. Arduino website says that there is no need to update the firmware if you have rev 0001.
I also replaced the avr folder of my IDE 1.6.4 with the avr folder of 0023. Also nothing...
I am out of ideas!