- I've experienced that same situation on older BMW's and apparently it was due to a severely drained battery. Haven't done it on an X5 yet (fingers crossed)

- I've found that portable jumper boxes sometimes just don't have the juice to do the job.

- No issue with jumping an OBD2 car, just follow the manufacturer's instructions and you'll be fine. (ie. Before disconnecting the jumper cables from your BMW, turn on the rear window defroster and set the blower to the highest speed; allow the engine to run approx. 10 seconds. This will prevent a voltage surge from the voltage regulator to the electrical accessories)