Mega2560 are notorious for USB/Serial problems. Hard to upgrade firmware. I have bricked a couple of them. The Rev 3 Mega2560 with the latest firmware are on the Arduino website, but who sells them in USA? Most vendors don't identify the revision number and I think a lot of old boards have been dumped on market.

 

You should be able to unbrick the bad boards using any AVR ISP programmer.

 

I tried AVR ISP programmer. No luck this time. Can't make connection with one, the other connects, won't accept bootloader.

Radio Shack is advertising Rev 3

 

Pick up the phone.. Try sparkfun.com or adafruit.com

 

That is weird. So the boards got “bricked” during a serial firmware upload? I don't see how the ISP could be disabled that way. Do you suppose the AVR is somehow damaged by something such as an ESD event, bad solder joint, bad power supply, bad flash memory, etc.? Those Mega boards are not cheap so it's a shame to have $100+ of useless hardware sitting around.