H E R E <-- Make sure to read the ADB Guide.
I recommend doing this with a Mobile just taken out of the box or with any Official ROM of its Service Provider. If you have already Flashed your phone with another ROM but its working fine, then you can use that /efs also.
Why the /efs folder?
This is a very sensitive system folder that contains Phone-specific information such as the IMEI (encrypted in the nv_data.bin), wireless devices MAC addresses, product code (also in the nv_data.bin), and much more. Often users trying to change product codes or trying to unlock the mobile will end up corrupting data in this location.
Why back it up?***WARNING: I take no responsibility to any damage caused by the methods cited and/or written here. Their sole purpose is to back-up data and not to alter in any way the integrity of the original files of the mobile***
What you will need:
- Rooted SGS II to get permissions as a SU (Super User) and perform the backup
- <- Busybox Commands(or Google them)
- Terminal Emulator by Jack Palevich (available from the market) <-Terminal Emulator or use ADB which is included in the SDK Development Tools
Here are 3 methods, I recommend that you do both Clean and RAW.
Basic, unix permissions lost :
su busybox cp -a /efs /sdcard/efs/
su busybox tar zcvf /sdcard/efs/efs-backup.tar.gz /efs
su cat /dev/block/mmcblk0p1 > /sdcard/efs/efs_dev-block-mmcblk0p1.img