Com port disappears when I apply power

I just created and uploaded a new firmware build for a diamond triple extruder (using the DiamondRepetierConfiguration.h file), but when I go to test it as soon as I apply 12v my Com port disappears, the arduino gets disconnected.

I currently only have only the following connected:
New extra stepper board - Aux 2 (44, 64, 59, 5v, gnd) and 12v, gnd near polyswitches.
DRV8825 stepper drivers
X axis stepper
Hot end thermistor and heater
12v at 5A connection only
2004 LCD controller

To be more clear, if I have the board plugged in with only USB I can connect to COM 6, but I get the "X commands waiting". As soon as I apply power I lose connection altogether.

If I apply power before I try to connect then the COM 6 option does not appear. It also disappears from the tools>ports option in Arduino IDE.

Any ideas?

I swapped RAMPS boards and everything started working. There must be a solder bridge or a bad component on my other RAMPS board. Strangely, the replacement RAMPS board does not pass through any power to the Arduino, whereas the other does. Granted these are a couple years apart in age, but I can't find any real difference between the two. Just means I'll be running this one tethered for now.

Also, though I was certain, even double-checked and triple-checked, the baud rate was set to 250000.

The new working Ramps sure misses the D1 diode. It is placed under the X-driver, so you might not recon it.