[AT&T, Rogers, Bell, Telus Samsung Galaxy S III] KevlarTheGreat:
How do we KNOW the IMEI isn't in the EFS partition? I don't think clockwork backs up the efs partition does it? I don't know for sure, but I don't think it does. When I look in the efs partition, I see /efs/imei/mps_code.dat which would imply that the IMEI is indeed located in the EFS partition. Instead of trying to back up your EFS by copying the files out of it, I think doing what someone else suggested and backing up the whole EFS partition as an image would be more thorough. As someone wrote earlier in this post, ADB didn't work for me to create the image and that may be why the EFS backup wasn't working for people. Using this method should work if the IMEI is actually located in the EFS partiton. I suggest people at least backup and if you get the IMEI problem, you have nothing to lose, so you might as well try restoring it. Her e are some instructions I posted in on of the T-Mobile threads.

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I did not originally write these instructions, it was this post. You can backup your IMEI by doing the following (this assumes you are already running a CM ROM where your external SD card is at "/external_sd" if it's located somewhere else in your ROM or you don't have an external SD, just replace that string with the correct one for your ROM (probably /sdcard):

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

This will NOT help if you already have a messed up IMEI. Neither the author of those instructions or I have had the corrupt IMEI problem and neither of us have been willing to try the restore part of it, but I did the backup part and it didn't cause any problems. Since this has a pretty good chance of working, since IMEI is (maybe?) stored in the efs partition, I think it would be a good idea for everyone to make a backup before doing any more flashing. If someone does this backup, then loses their IMEI, please PM me to let me know if it works, then I will try to write an app to automate this.

Copy the efs_backup.img off your sd card onto your PC for safe keeping. Do NOT let anyone else get this image from you.

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Here are some Theories:

Maybe Broken MTP storage on early CM / AOKP builds could have corrupted partitions on internal memory? I know there were some builds on other phones that would do this and I know MTP storage was kind of working, but not quite....

Tons of people seem to be using the wrong version of clockwork. Need to clean up forums so people use the correct version for their device.

People may be flashing the wrong ROM for their device.

I doubt it's CWM causing the problem unless people are using it to manually format partitions / SD Card. Maybe it's formatting the internal SD/NAND? I have a feeling the few people that have taken the risk to install TWRP are advanced users and just haven't made the same mistakes as the less experienced users. There are 100's of people using CWM and the main developers of cyanogenmod recommend (on the T-Mo JB CM10 OP) to ONLY use CWM until this is figured out. Personally, I trust CWM and don't trust anything else, you make your own choice. It's tried and true. But who knows at this point, I won't say that any more if I have the problem.....

Some people are saying they think the carriers are doing it on purpose. If they were doing that, it would be more consistent and everyone would be losing their connectivity.

Could be permissions on the IMEI file (as someone here stated already). Did people run Fix Permissions? Maybe fix permissions actually screwed it up?

Could be ROM manager telling recovery to do the wrong thing...?? I doubt it, but it's still possible.

I know, I know, go to the link in the OP. I got it, I'll look there, but it doesn't hurt to discuss theories here either.