[Q] How to back up imei/efs folder? topic

There's the thing: I tried to flash my Sony Xperia M C1905 to CM12 and I got stuck on the Sim Card Missing problem, so I have to go back to stock 4.3 version. I want to go back to use the CM12, but I can't find the EFS folder to back up. I have used the root explorer and es explorer, but don't find anywere.

Pls, help. ^^