[Galaxy Note II General] dr.ketan:
What is EFS

This part of the device contains some important and some basic info related to our device identity like Bluetooth MAC address, wireless devices MAC addresses, product code (also in the nv_data.bin) and the very main IMEI address (nv_data.bin), programming parameters for the device such as your account information (phone number, etc), data provisioning parameters, and a whole bunch of other things. So you can guess it's will be different for each device and one can't use others.

How it can corrupt :

- Any bad flashing, mishandling system modification can cause this.

- Also It can be corrupted by downgrading stock ROM. If you flash any old ROM (older to first ME6) on newer ROM (ME6 or any other rom released after ME6) it can corrupt EFS. Basically newer ROM believed to have new kernel that changes EFS data which older Modems can't read and results to fail in reading IMEI.

Impact on device/How I know I have corrupted EFS

When there is EFS partition is corrupted, device loses IMEI and due to this device can't gets connected to network. Baseband info also sometimes shows unknown.

-Check your IMEI in setting / by dialling *#06# if it is null OR if it starts with 0049** then IMEI is generic and corrupted. In Normal case IMEI which you can see in device setting or dialing *#06# is exactly same you can see on bill/box/sticker under battery compartment

-If you are unable to connect to network in spite of correct IMEI, chance are

A. Your device is Locked to specific Network OR

B. Your device is banned due to tempered IMEI

-If your AP,CP,CSC all are correct and even you are getting error (unknown CSC XXX) on kies update, it may be due to corrupt Product code in EFS. Read This for detail.

What care should i take

Before flashing anything, first thing you should do is to make EFS backup. Every person who have rooted device should make EFS backup earliest and place it to safer place on PC and /Cloud.

If you are on older ROM, make EFS backup on older ROM as well make another after upgrading ROM too.

If you have already updated to newer ROM, make backup on current ROM.

Here is different common methods for EFS Backup/Restore

[There are hundreds of tools available on play store and don't use blindly, as unsupported tool can cause EFS partition failed to mount and to recover it need to use adb shell and more. Better use one of the below mentioned time tested tools]

You must have device rooted. Read post #2 of this thread, how to root Note2

you can use any of method said below.

1.With Aroma installer - No need to have PC, need CWM/TWRP installed. Here is Other Tool I recommend to must have for Newbies, Which also have many other options.

2.From Device itself - No need of PC, just requires installing application from play store.

3.Philz recovery - Also you can find this option on other CWM/TWRP, but i have tested only this.

4.EFS Backup/Restore Tool - you needs to use PC to work this.

How to recover lost IMEI that happens with downgrading ROM

If you have lost IMEI just by downgrading (as said by above by flashing older ROM), still you can recover IMEI without having EFS backup.

-Again upgrade ROM will restore IMEI OR

-Flash latest Modem. You can find it here.. You can download MG1 modem and flash using PC Odin with 'Phone' tab instead of PDA.

Remember above said works only if IMEI is corrupted by downgrading ROM only, if it was corrupted by other things, this won't be helpful.

Also Read This, if you have issue restoring EFS OR device stuck to factory mod.

Hope this will find helpful to all newbies regarding question about EFS and IMEI problem. I have posted this to general section so newbie can post to it if they have any queries.