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What Rick and most others said...

On a scale of 1 to 10, 10 being the most probable cause of your battery drain, I'd give the MSD box a 2.

There's an easy way to check though...

Disconnect your MSD box ground wire(s) and with the car off and momentarily touch the wire(s) back to ground. If you've got enough drain that it kills your "good" battery in 1 day then you'll definitely see a noticeable spark when brushing the wires against ground.

The professional way to do it would be to again disconnect the box's ground wires with the car turned completely off, in the battery drain mode. Place a voltmeter between the ground wire(s) and actual ground. If you've a battery drain you'll read a voltage. The amount of volts read will be proportionate to the amount of drain.