So after some more digging, I've found the cause to be my slimline USB CD/DVD drive. If I have it plugged in and attempt from any mode other than P0, the CD/DVD just makes a click sound and power is cut.
As soon as I disconnect the USB drive, the M2 can boot from any setting (P1,P2,etc). The CD/DVD drive seems to be the only USB device doing this. The system boots fine if I leave my USB GPS receiver connected.
So my M2 is fine but I don't know why the USB CD/DVD drive would be causing this. Gonna to some more digging/searching.
If anyone has any idea why this would be happening, any advice is welcome.
I'll keep this thread open as a reference for others who may experience something similar.