[AT&T, Rogers, Bell, Telus Samsung Galaxy S III] getsaf:
I believe this is the most important question that can be asked but it seems to have been looked over.

I know I've seen people say that the EFS Backup "App" doesn't work, but I'm not terribly surprised by this. Backups don't get much more rock-solid than a 'dd' backup.

(don't remember which thread this was from but it was posted by id_twin)

Backup by typing this in the terminal:

dd if=/dev/block/platform/msm_sdcc.1/by-name/efs of=/external_sd/efs_backup.img

Restore by typing this in the terminal:

dd if=/external_sd/efs_backup.img of=/dev/block/platform/msm_sdcc.1/by-name/efs_sdcc.1/by-name/efs

For those who are not familliar, this basically backs up a partition by reading directly from the storage device and outputting the byte-for-byte into a file that can be copied off for safe keeping when done. The restore process does the opposite, but writes byte-for-byte the entire partition block back to the partition area exactly the way it originally was pulled.

The important thing to know here is that most backup/restore apps will copy "files" out of a partition and into a different storage format than the partition such as tar or zip. To restore from one of these kinds backup apps, you would have to have an existing properly-formatted partition ready and mounted to extract the files to BUT when you're done extracting, the end-result is that you would have a different partition with the same files but with slight differences, such as the fact that the re-extracted files would likely be written to different areas of the partition than the original partition you extracted from, therefore your restore would NOT give you the EXACT same partition that you started with, just one that appears the same when mounted.

THIS kind of backup (dd backup) is a bit more awesome in that it will perform a partition-level backup which is the raw filesystem and all, it basically reads the 1's and 0's from a particular sector of your storage that includes the filesystem headers and filesystem structure and all in the backup. This means a couple of things. DD doesn't care (or know) what the underlying file-system is, in fact, there doesn't even need to be a filesystem present, it's just bytes in a block on a drive. This means that restoring THIS kind of backup will restore the base-level filesystem in-tact byte-for-byte. It's an exact mirror of your partition. These backups are so solid that these kinds of backups can actually be mounted as if they were a drive-partition on their own because technically, they ARE exactly a partition.

That being said, I really would be shocked if someone couldn't restore their EFS partition this way. I have backed mine up with this method but have not had any IMEI issues as of yet. If someone would be willing to share their DD backup with someone who has lost their IMEI as a proof-of-concept just to let the rest of the group know if this backup/restore method is effective that would be awesome. I have to admit I'm not terribly fond of handing out my IMEI to anyone, but it would be a great test for someone who has TWO GS3's handy (one with a bad IMEI and one good).

Hope this post helpful more than it is boring. I hope this gets figured out soon. I'm still a bit scared to flash another rom for now.