Take a look at the SD card adapter. Make sure there are no blobs of solder between pins. Double check where you were plugging the adapter in. It's very easy to get moved over one pin the wrong way on these boards.
The computer should power the Mega + Ramps just fine from the USB. It will not run the printer this way, but it should power up the Mega so it's recognized. It's probably safer in your case to check the SD issue out this way than with the high power supply connected.
The computer should power the Mega + Ramps just fine from the USB. It will not run the printer this way, but it should power up the Mega so it's recognized. It's probably safer in your case to check the SD issue out this way than with the high power supply connected.