avrdude: Yikes! Invalid device signature.
Double check connections and try again, or use -F to override
The reasons this error is produced...
1. Auto-reset not disabled. AVRDUDE is communicating with the Uno bootloader instead of the Arduino ISP sketch.
2. Wrong processor. AVRDUDE is trying to communicate with a 328 but is instead communicating with a 2560.
3. Wiring. The wiring between the programmer (the Uno) and the target (the 2560) is not correct.
4. Clocking. The target must have a working processor clock.
It [the LED on pin 9] no longer blinks, it just keeps oscillating in and out.
Indicates that auto-reset has been disabled so we can cross off #1.
You have the correct board selected so we can cross off #2.
You are confident the wiring is correct so we can cross off #3.
We're left with #4. What is the provenance of the processor? Is it fresh from the factory?