[Galaxy S II General] jcrompton:
For those of you that don't think you need to read this - you are wrong! I have flashed 100's of ROM's with no issue until the end of last week.

MODERATORS PLEASE STICKY THIS IN A PLACE WHERE EVERYDAY USERS THAT FLASH ROMS WILL SEE IT

I uneventfully flashed a ROM (not important which one because I don't think it has anything to do with particular ROM) and all went well. I went to bed with a working phone and the somehow a reboot happened and awoke with a phone with no signal. The error was saying that there was no SIM card and / or No Network Available.

I finally used HC-kTool app on the Market (used latest version) to Restore an efs.tar file and my phone started working but I had a generic / fake IMEI number -- 004999010640000

If you have backed up your /EFS folder on the root of your phone then all is well. The easiest way is to use Hellcat's HC-kTool to back up your EFS and save a copy on your sdcard and your PC and that way you will not have to go through my misery.

However if you have not BACKED UP your EFS files then you will have issues like I did

Ok so here is what I can tell based on my experience:

Look for any files or folders on your sdcard that have EFS in the name. You might get lucky and look in the /EFS folder on your phone. If the nv_data.bin file has been corrupted the .nv_data.bin.bak might still be OK. If that is the case you may be able to follow this guide: http://forum.gsmhosting.com/vbb/f606/samsung-android-repair-efs-imei-repair-without-box-1348192/

What won't work is any sort of simple copying nv_data.bin from the sdcard to the /EFS folder using Root Explorer - I spent almost an entire weekend trying this.

If your /EFS folder .bak file has been corrupted then in my case I found a folder on my sdcard named EFS_BACKUP that I assume that Clockworkmod has created during some of my ROM flashes (somebody correct me if I'm wrong because otherwise I have no idea how it got there).

In the EFS_BACKUP folder I have a number of files form different dates (I assume of flashes) labelled efs_xxdate.tar.gz

btw you can use these or any .gz file when using HC_kTool tool simply by renaming the file (long press with Root Explorer) to just a .tar file (delete the .gz). HC_kTool uses a file named efs.tar on the root of the sdcard to restore.

In my case I had to extract one of these files by long pressing in Root Explorer and choicing extract. I then went to the extracted folder on the sdcard (using Root Explorer) and found a folder named efs_xxdate and within that a folder named EFS

... again don't get mislead -- simply copying and pasting that file or any other to the root /EFS folder will not work!!

I noticed that there were multiple files in the extracted/efs_xxdate/efs folder but the dates were different. The nv_data.bin and the nv_dat.bin.md5 files were dated the same as the date of the folder BUT the .nv_data.bak file was dated Dec31, 2010

Here's how I restored that file:

1) I extracted the .tar.gz file as above

2) Went to sdcard\extracted\efsxxdate\efs folder and copied the older dated .nv_data.bak to the root of the sdcard using Root Explorer (long press, copy, then paste)

3) Then renamed the .nv_data.bak file to nv_data.bin on the root of the sdcard

3) Then followed these instructions from line 14 on in this post .. http://forum.gsmhosting.com/vbb/f606/samsung-android-repair-efs-imei-repair-without-box-1348192/

14 = at command prompt [ C:\adbtools\ ], type adb shell

15 = type su

16 = type cp /sdcard/nv_data.bin

17 = type rm -rf /efs/nv_data.bin.md5

18 = type exit & exit

19 = REBOOT YOUR PHONE

I'm not sure why this works but EVERY combination of copying and pasting nv_data.bin , .bak, or entire EFS folders suing Root Explorer does NOT work but this above technique did work for me ! Somehow having the .md5 file recreated is the key ??

I hope this helps someone.

BACKUP YOUR EFS -- Hellcat's HC_kTool seems to work great

Thank the DEVELOPERS