What is EFS? It is a place that stores your Android smartphone important data like Wi-Fi, Bluetooth MAC address and IMEI (nv_data.bin). Do take note if you are a frequent ROM / firmware flasher or rooted your Android phone. Installed unstable poorly cooked ROM might cause EFS partition to be corrupted or missing. Which will cause you fail to make voice call due to missing IMEI number. So better to make a backup of your EFS partition. The easiest way to backup EFS ~ just copy the whole EFS folder with Root Explorer to SD card. But some said this method is not good during EFS restore. So better use EFS backup / restore tool like kTool by Hellcat. kTool will backup the whole EFS partition (including files read / write permission) for you with 1 click. Note – root access is required.