Samsung-a520f-u4-efs-eng-file -

: This stands for Encrypting File System . This partition stores the device's most sensitive data, including its IMEI number , MAC address, and network calibration settings.

: If the EFS partition becomes corrupted during an update or a bad root attempt, the phone loses its ability to connect to cellular networks. samsung-a520f-u4-efs-eng-file

: If your phone has updated to U5 or higher, this U4 file will be rejected by the device and will not flash. : This stands for Encrypting File System

: Short for Engineering . These files are often pulled from factory testing units. They are "unlocked" or "simplified" compared to consumer files, making them easier to use for repairs. 2. Why is this file used? including its IMEI number