[Galaxy S III Q&A, Help & Troubleshooting] kaynpayn:
I'm just writting this because i've recently had this problem. I managed to sort it out by my own but didn't really find it written anywhere, just bits and pieces scavenged from so many pages that might point to a solution (granted, part of my problem was also finding what was really my problem since the phone stopped working so fast i didn't really had the time to get what happened)

I was trying out swap for my S3, i had created a swap partition in my external card and then followed a guide here from xda to use an app called swapper2 (from market) to enable swap. I must have done something wrong because what it did was format my /efs partition instead as if it was a swap partition. Phone stopped working on the spot, obviously, and wouldn't past samsung's bootscreen.

Also, keep in mind while this worked for me there might be other ways of doing this which may be simpler. This is just what i figured for a solution.

When this helps:

- you accidentally formated your /efs partition

- you have a backup from your /efs partition, files copied with root explorer but not an image you can just flash back to the partition.

- your phone boots to download and to recovery but not to main system (which every tutorial i found seemed to assume to be able to do, i just figured their /efs was not as screwed as mine was)

- your recovery won't mount the /efs partition because it expects it to be ext4 but it's formated to something else and it showing the message:

"e: failed to mount /efs (Invalid argument)"

What doesn't work:

- any kind of standard solution like reflashing a stock rom with odin, with or without .pit file, clear EFS, whatever. It will flash, just won't solve since, apparently, nothing touches the /efs partition because it's so sensible. Even custom recoveries do mount it and apparently format and replace every single other partition but the /efs, which is why i had to do it manually.

Solution:

- simply format the /efs partition back to ext4 and copy your backup files there :)

What's needed:

- phone drivers installed on the computer. If you had kies installed, this should be covered already. Think you can't have it running while doing this though. Also, for what's worth, i've done this with windows 8 x64.

- updated android sdk tool, mainly adb. If adb is saying device not connected or not found, you may be using an outdated adb.exe (i was). I used adb.exe that came with this software here:

http://xdaforums.com/showthread.php?t=1308546

- root. If your rom isn't rooted you can always flash CF-root from odin in download mode.

http://xdaforums.com/showthread.php?t=1695238

- busybox, am not really sure if i actually got to need this, think so for the formating tool. Can't hurt to have more options though. This is how to install:

http://www.omappedia.com/wiki/Android_Installing_Busybox_Command_Line_Tools

- custom recovery that allows root through adb, i used Philz which is pretty awesome

http://xdaforums.com/showthread.php?t=2002953

- backup files from your /efs. I had mine unzipped and copied to my externalsd while in windows. They were in an efs folder in the sd card. You can push this through adb to the phone later, but i found this way simpler.

How to:

- get into recovery (vol up+home+power button) and plug your phone to the computer. You'll hear an usb connected sound from windows after a bit.

- in recovery, navigate to Mounts and try the "mount /efs". It'll fail. Now, navigate to advanced and "view log". You'll see below that the phone tried to mount a partition and failed, in my case, it had unsucessfully tried to mount /dev/block/mmcblk0p3 which is the partition /efs uses. Take note of this partition name.

- On windows, open a command line (type cmd on you start menu), navigate to where you have adb.exe and type:

adb shell

If all went well, command prompt has changed. You can now issue commands directly to your phone from terminal.

- type "su" to get root permissions. Your command line should change to something like root@android. Before i had this working i needed to use "Fix permissions". It's an option in the recovery menus on the phone.

- this step is optional but i did it anyway. I created an image of the partition i was going to mess with. If anything went wrong i could always flash it back even if it was broken. By the way, this is what i should have done in the first place instead of just copying /efs files. Would have made the process so much simpler... also, do this if you ever get the phone back working again. Type:

dd if=/dev/block/mmcblk0p3 of=/externalsd/efs.img

The /externalsd/efs.img is just a place you can save stuff to, like your external sd card. You can check what's your mounting point for the card with the command "mount" and look for something relevant like "externalsd".

Also, you may want to copy that efs.img to somewhere else from your phone (like your computer) for safekeeping. Literally remove card from phone, put in computer and copy files there. Notice you may need to reboot phone to recovery if you remove your card so it detects and mounts your card again. Sure, you can also do that from the command line, but it's just easier to reboot the phone.

- Now, we'll format the partition to something your phone can mount (ext4) and is expecting. Be careful, by doing this, you're actually erasing what's left of your partition and replacing by something clean. Type:

mk2fs /dev/block/mmcblk0p3

If all went well, you should see some kind of small report about what was just done.

- To mount this we'll need a mounting point. Type:

mkdir /efs

this will create an /efs folder in root, if it doesn't exist already, and to mount, type:

mount -t ext4 /dev/block/mmcblk0p3 /efs

- if all went well, you now have access to the /efs folder and can copy you files back:

cp -r /externalsd/efs/* /efs

where externalsd was my external sd card mounted on my phone.

- Check if your /efs folder has it's contents properly

ls /efs

I had to reboot the phone to recovery once more at this point and use the "fix permissions" option again.

After this, my phone booted up normally again.