SOLVED : also without sending it back, I must add. I have posted this solution in couple threads and have solved many people's problem. That's why I'm starting this new thread.
Thanks to the user DivenDog. His method includes opening the device. I must admit that I found out this technique after reading his post.

What happened
I got the drone delivered before work. i was so eager to fly it. I did not charge the battery which is low.. I flew it. I did not mind the low battery thinking that it has the safe return feature. I was in the road in front of my house and I found that a car is leaving the parking and my drone is in automatic descent. I did some manoeuvres to avoid the car and crashed the drone in the building some 30 feet above. It came down scratching the building all the way. I rushed to it and turned it off. I found some scratches. Propellers are ruined. I thought I got lucky and went to work.
After coming back and tried to fly and I got this error. I tried pushing in the gimbal cable, power cycling it many times, etc. I tried to calibrate IMU and It was giving the error "Unknown error" at 35% the first time and at 2% after that.
I called DJI US though I live in Sweden.. It's already evening. After waiting for 42 minutes, I talked to the technical support people and they said I have to send the drone in and it will take 4 to 6 weeks for them to fix it as they have queue. I have trips planned and I decided to open it tomorrow as I do not have the tools to open it today.

How I fixed
Since, I decided to open it the next day, I looked for some help and I landed in this thread . I decided to try smacking the aircraft. It's not some random decision. I will explain why in the "What I learned" part. Since the error is caused by the aircraft hitting the front, I smacked thrice on the back of the aircraft. I turned the aircraft on, went to the sensors page and sensed (explained how in the next section) that the IMU calibration will go through and it did. Then I took the craft and flew it.

What I learned
I also do mobile development and I know how the digital accelerometers work (A metal on a diaphram moves in a magnetic field to generate current) and have seen raw values of gyroscope, accelerometer reading of mobile devices. when I went to the sensors menu in the advanced settings, I saw this.



I did this, when I could not calibrate(the screenshot was taken later). I saw the value -16 in the accelerometer's X. Initially I didn't care much. After smacking (with the device off), it was showing the normal values (around negative 1 to positive 1), as shown in the screenshot. Then the calibration went through.
I know that the gravity affects these values. So, rotating the craft in different direction changes these values in different axis. The X though was not changed. It's stuck. This made me think that the metal on the diaphram of the acclerometer is stuck and I need to smack it in the opposite direction to release it (and not make it get stuck in the opposite direction). So there is a chance it's stuck in either of the six directions and based on that you have to decide where to smack.
Solution: This is where you need to smack, (might be a combination of smacks, if it's stuck in more than one axis) based on the acceleromater values.
low negative X - back (as the metal is lodged on the back of the craft by got hit on the front)
high positive X - front
low negative Y - left (nose pointing away from u)
high positive Y - right (nose pointing away from u)
low negative Z - bottom
high positive Z - top

I hope this will help. Good Luck.

This was extremely helpful!

tomas3 Posted at 2016-5-1 07:45
This was extremely helpful!

Hey, thanks for this youre amazing, my drone is up and running!

That's great you were able to resolve it that way.
In some MC Date error cases you can also downgrade/upgrade the firmware. This is usually only after getting an error after updating.