How to Fix E: failed to mount /efs

How to Fix E: failed to mount /efs (invalid argument) in All Samsung Galaxy Phones.
Self Tech Genius > Android > How to Fix E: failed to mount /efs (invalid argument) in All Samsung Galaxy Phones.
If you are a hard core android user who loves flashing ROMs and tinkering with your device limits, all this can be a fun experience, but as you probably already know it does not come without some possible risks if you are not careful. Having a bad EFS folder can be your worst nightmare yet which cause many sleepless nights. Troubleshooting this kind of problem could prove much more problematic than reviving a hard bricked device. A damaged EFS folder will cause your device to lose any kind of mobile networking capabilities leaving you unable to receive any calls, messages or access the internet. In Samsung devices, the EFS partition /EFS folder houses critical information about your device core networking data: IMEI, WIFI, Bluetooth, MAC’s, radio info and others.
Most devices with a bad EFS folder/EFS partition will show “Blank IMEI” “Null Null” “Not registered on network” and probably lose the Baseband version as well. A device IMEI is the unique ID assigned to every GSM phone and which can be used to validate a phone to the cellular network among other purposes.

How to Fix E: failed to mount /efs (invalid argument) in All Samsung Galaxy Phones
I recently received a Galaxy S2 i9100 for repair with Blank IMEI number and No baseband version and no network. With no previous backup of the EFS folder or a backup of the IMEI, I graced myself for the task ahead.
The most recommended program; EFS Professional and EFS backup/restore tool could not repair the corrupt EFS partition or restore correct IMEI data successfully.
Strange enough, the device got stuck at the Samsung boot logo and could not boot all the way up. Restoring device to factory and clearing cache in recovery did not help either solve the boot loop.
Then, I encountered this error in stock recovery. ” E: failed to mount /efs (invalid argument) “
I tried upgrading the device from Ginger bread to official Jellybean 4.1.2 ROM in hope that a complete wipe would enhance the chances of recovering the EFS folder data to hopefully get the network working again. I tried different Odin flashable ROMS but after Odin finished with no errors, the device still could not boot past the Samsung boot logo, kept restarting frequently. Soon, I realized that no ROM flash, kernels flash or CWM jigs could resolve this error until I got repair damaged internal data partition.
I scoured thoroughly on the internet but information about fixing this error was scanty and I could not find a working solution to fix my i9100 bad EFS partition.
When the last drops of hope seemed to dwindle away, I stumbled upon some information about this error but which only applied to another device. So I did my diligent research on getting the same to work on my Galaxy S2 and possibly every other Galaxy device with this problem.
So today am posting this solution in the hope to save many others the agony of getting stuck with a seemingly useless device.
NOTE: This solution worked on GS2 i9100, if you are having the same problem with another Samsung device, the same could work but you MUST change the EFS mount point I.e ‘mmcblk0p…’ part. For instant, Galaxy S2 i9100 EFS mount point is mmcblk0p1, for Galaxy S3 i9300 use mmcblk0p3 , Galaxy Note 3 use mmcblk0p3 as well and so on depending on your particular device partition table. So look around for the correct EFS mount point for your device.

How to repair and restore bad EFS partition folder.
Instructions:
Download and flash official firmware for your phone in Odin. I flashed the i9100VJMS1 4.1.2 Jelly bean.
Flash a CWM recovery and root your device in recovery. For S2 I used PhilZ CWM.zip recovery which is root ready for your installed firmware
Install ADB on your computer. See How to Install ADB, Fast boot tools and drivers the easier way.
Boot your device into recovery