Hey folks - I know this is an older post - but in case others are looking for a possible solution in the future....
My new Spark arrived this week. Straight out of the box it had "Main Controller Data Error", and the IMU failed to calibrate. It failed at step 1 of 6 with the same -71 error code. DJI wanted me to return it. A very slow process where I live (7,100 islands) and the week before Christmas the deliveries are delayed more than usual.
No amount of searching came up with a fix - I downgraded and upgrade FW, all the usual tricks......
It also showed an Accelerometer error - that was the clue to the issue.
An old post in the DJI forum (Phantom I think) - was about an accelerometer failure and resulting MC Data errors.
I went into debug mode in DJI Assitant - found my Y axis accelerometer was stuck on 16 when in IMU calibration - regardless of moving the AC. It was stuck - same as the Phantom forum post. The other Axis were under 0.2 or so.
These units have fragile internals like these sensors. Without any FRAGILE stickers on the packaging - who knows what happens to them in transit. Mine was most likely dropped, hit or kicked
2 sharp finger taps (not too hard) to the right side of the AC body (it was a +ve Y axis reading) - it free'd the sensor. Then the IMU calibrated as it should. Have completed my first successful flights with no issues. No returning the unit and I get to use it over Christmas as planned....
Local DJI guys are aware - if I have any more problems it'll go back to them (and I wait a few weeks

). But before you send back a unit with a IMU 71 error - worth a couple of quick taps (once you know which axis is stuck).
cheers