Every Samsung Android devices comes with the infamous \EFS folder. So, what EFS folder have and why backup is very important here? EFS folder is the very sensitive system folder that contains Phone-specific information such as the IMEI (encrypted in the nv_data.bin), wireless devices MAC addresses, product code (also in the nv_data.bin), and much more. If this thing not handled properly, can turn your device completely useless as it will no longer be recognized by your carrier. Problem is, flashing custom ROMs sometime can corrupt contains within EFS folder. That’s why backing up EFS folder is very important if you