View source for EFS partition

=== EFS ===
The EFS is a special partition dedicated for your phone's info like radio signal info, imei, serial number and lots of files revolving around your SIM card and Wifi, Bluetooth (this includes your MAC address for all the radios of your phone) in an encrypted format at the file system level stores. 
Imagine the EFS as a big folder containing all of the important stuff that makes the "phone" part of your phone. So it is very necessary to backup EFS before flashing ROMs.