Encrypted File System (EFS) | KingoRoot Android

What is EFS?
The EFS refers to a folder called EFS which can store information like meid, imei, serial number, config, diag settings and radio settings, etc in an encrypted format at the file system level stores. So it is very neccessary for us to backup EFS before flashing ROMs.
How will you lose the EFS folder?
As far as I known, you always lose your EFS by mistaken operations. For example, flash ROMs or wipe data using an uncorrect method. Even wrong power on & off ways like taking out the battery to force shutdown as starting could corrupt your EFS, too. Rooting and flashing ROMs usually have no influence on EFS data, but you know everything can happen in the real word. There are some ways you can check whether your EFS folder is lost: inputting *#06# on dialler, going to settings/about phone/status or checking mac address.
What results if you lose the EFS folder?
The loss of the EFS folder may bring about an incredible disaster on your device such as no imei information, base band unknown, no radios and so on which mean your device will no longer be recognized by your carrier. After knowing that, won't you back up the folder?
How to back up the EFS folder?
There are too many tools you can use to back up the EFS folder. All of them work in a similar way, so we will just introduce one tool called Custom Recovery here. Everything has risks. we are not responsible for  the following method. Please act with caution!

Bofore backing up the EFS folder you need make sure your Andriod device is rooted. Then, install Custom Recovery on your device. Open the Custom Recovery, select the Backup option, and go on select the paritions containing EFS, then "Swipe to Back Up" to begin the backup process.

That's all. Now you have a clear understanding of EFS!