Rooted your Galaxy S4? Then stay with me on this guide, else feel free to skip this. Without root access, you won’t corrupt or delete EFS. So you are safe.
EFS folder is a place that store all Samsung Galaxy S4 important data like Wi-Fi, Bluetooth MAC address and IMEI (nv_data.bin). Therefore, don’t play with system level folders especially EFS folder if you don’t know what they are. For you info, your Galaxy S4 can’t make voice call if IMEI number is missing. Therefore, it is better to make a backup of Galaxy S4 EFS. Why? Install unstable or poorly cooked Galaxy S4 ROM might cause EFS partition to be corrupted or missing too.