The most common cause is user error. When flashing custom ROMs via custom recoveries like TWRP or CWM, sometimes the "Wipe Data/Factory Reset" option is interpreted too aggressively by the recovery script, or the user accidentally formats the wrong mount point. If the /EFS partition is wiped, the data is gone.
: The device may get stuck on the Samsung logo with an error like E:failed to mount /efs in recovery mode. n7102 efs file
A reader, “Mike from Sydney,” wrote in: The most common cause is user error
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