I have a problem again with an Arduino Mega 2560 board (SainSmart). The IDE gets stuck at the "Uploading to I/O board..." message. The bargraph shows around 95% but then there is no further progress. I tried this with Windows 7 64bit and Win XP, with 101 and 0023. Uploading is impossible. Board type and COM port are correctly set in the IDE. Any ideas to solve the problem?
Do you have another Mega board to test with? That way we can tell if it's just this specific board. Is it timing out? Something like...
avrdude: stk500_2_ReceiveMessage(): timeout
It could very well be a driver issue. Give this a go:
1) Go to System Preferences -> Network
2) If it asks you to add the Arduino (modem device), say cancel
3) Remove the Arduino device from the list if its there
4) Try uploading a sketch!
1) Go to Device Manager
2) Find your Arduino in the list, go to its properties and update its driver manually, using the .inf file which came with your Arduino (0021+) IDE
Also, make sure you're not sending
!!! via serial at any point in your sketch as that initiates a special monitoring mode baked into the bootloader.
Also, take a look at the official Arduino troubleshooting guide.