[Galaxy S III General] danarama:
I see this question / problem comes up a lot. Rather than retype my answer every time, I have created this thread so I can easily just give people a link to it.

Scenario

You've flashed a 4.2 ROM for the first time coming from 4.1 but Titanium Backup cannot see previous backups on /sdcard and file managers cannot see your /sdcard data. You are concerned you have lost your data.

Or.... You're looking in the storage menu and you notice the total of the "used" should leave you more "free" space

Yes! Have I lost my data?

No

Background info

Older android devices have the below (simplified) layout.

/system - Where the ROM goes (internal memory)

/data - where your apps and app data goes (internal memory)

/sdcard - your sd card (removable media)

The S3 works differently

/system - Where the ROM goes (internal memory)

/data - where your apps and app data goes (internal memory)

/sdcard - your sd card (internal memory)

See that last part? The /sdcard is now on internal memory too. Samsung have actually used the clever FUSE file system, to create the /sdcard mount point (exFAT) inside /data (EXT4). So /sdcard is actually mounted to /data/media. That's how "internal storage" goes down if you install an app or if you take a photo. It all goes into /data at the end of the day.

Back to the problem

With 4.2, /sdcard is no longer mounted to /data/media

Now, It is mounted to /data/media/0

When you use a file manager (or titanium backup) to look in /sdcard, it looks pretty much empty (which it is). Because you're looking in /data/media/0 but all your data is in /data/media

So what to do?

Quite simple and very easy.

Use a file manager with root explorer capabilities. I prefer root explorer but ES* and Solid should manage this too.

Go to /data/media

You should see:

0

legacy

obb

Along with all your other folders. Legacy and 0 are the same. Leaving 0, legacy and obb UNTOUCHED, MOVE all the other folders (which you will notice are all your missing folders and files) into the 0 folder. The only folders that should be left behind are the 3 mentioned above.

That's it. Done.

Anything else to be aware of?

Yes actually, many older recoveries still probably see /data/media as the location of /sdcard, so nandroid backups may end up there instead of /0. For example, often you end up with /data/media/clockworkmod instead of /data/media/0/clockworkmod. Some recoveries have a setting to change the location to /data/media/0 (Such as Philz recovery)

Because the Settings > Storage > sdcard shows the space free and used for /data as "sdcard" but only looks in /data/app, /data/data and /data/media/0 to get the breakdown of what is using the space, you will find that anything just in /data/media will show aup s used space, but the breakdown of apps, pictures, media etc will not show these files so it appears that they do not add up.

Example...

You have:

1 GB apps

1 GB Pictures and Video

4 GB Nandroid (Misc) - but this is in wrong place /data/media

Used space will show 6 GB

But breakdown will show

1 GB apps

1 GB Pictures and Video

0 GB Misc

So it looks like you lost 4 GB. It's a good idea to check /data/media every now and again to ensure nothing goes there by accident.

I hope this helps.

*ES has updated it's UI and isn't as obvious as before how to get to /data/media so see below:

Open ES > menu > tools > root explorer

When prompted, accept root access request

At the top of the screen (you've probably defaulted to sdcard) press the "/" button to get to the root of your device. From here you can browse to /data/media