Hello,

I would like to verify something before I modify our reprap board.

I have a pre-assembled reprap 1.4 (with power source) and an arduino mega (with USB connected constantly). Is it required to remove the D1 diode, as there are now 2 power sources for the arduino? And if yes, can I disable it without cutting out the diode?

No, you can leave the diode D1 in, as long as you do not also connect power to the DC-jack of the Arduino Mega.
The Mega has a special circuit built-in to arbitrate between the 5V coming from USB and its own (weak!) 5V regulator.

OK, thank you!