[Galaxy S III Q&A, Help & Troubleshooting] whitepaw:
I'm not sure what exactly "System Data" is collecting.

On my phone System Data is 3GB (might be some zombie files in there which aren't used anymore).

It definitly is caused by apps you install.

The "System Data" part is not one explicit folder as far as I understand, but the collection of multiple folders.

Have you installed an android 4.2 ROM?

If yes:

I had a problem where my storage was nearly filled completly and I was not able to find where the data is.

I noticed, that after installing the 4.2 Leak, the MTP Mountpoint was moved to another folder inside the "normal" storage partition cause of the "emulated" multiuser support. (I don't have my phone connected right now, so the foldersmight be wrong) As far as I remember, the original /storage/sdcard was moved to /mnt/shell/legacy (which contained my previous sdcard files and a folder called "0") and the new /storage/emulated/legacy and /storage/emulated/0 (first and only user for mobilephone android 4.2) where linked to that folder "0".

That means the rest of the files in /mnt/shell/legacy where "hidden" and I was not able to find them via windows explorer and some android storage tools. After I moved the files from /mnt/shell/legacy to /mnt/shell/legacy/0/old via adb, I was able to detect them correctly and remove unwanted files for cleanup. However I'm not quiet sure if that folder counts, cause it is not part of /data.I tend to say, that it does not count.

If <4.2 or if the above is not the case:

I think 1GB isn't much. I might be wrong and maybe it's a good idea that someone else can share the "System Data" size of their diskusage result. I myself might not be digging deep enough into android filesystem management, but it depends on what "System Data" is showing. if it's really showing every little system data, then it contains the apps, the configurations, the databases, the recovery, the rom (which alone is around 500MB), the sounds, the wallpapers...etc.

I think it also depends on the device what data is shown here, you can't compare the result of your device with for example a HTC Device cause it might have a complete difference partition and mount table.

The best idea might be to connect the phone to the Computer, start an adb shell and get root (I hope your phone is rooted) by typing:

su

You might need to allow that on the phones screen.

Now type:

df -h

This will tell you which mountpoint takes how much space.

Example of another device:

/dev/block/mmcblk0p12 3.0G 958.2M 2.1G 31% /data

I'm not sure about the output on android, but it should show the mountpoint (a folder without /dev)

If you can only see something with "/dev" it's not the mountpoint but the device itself. You can find the according mountpoint by typing "mount".

Maybe you can find your 1GB here.

Change into each directory you want to check (type: "cd /data" for example) for the size and execute:

du -sh *

and you will get a list of each folder and file in there including it's size (not only the folder size (which will be displayed with "ls -la" to 4KB (cause the folder itself isn't big), but also the files within that folder).

Now go on and dig deeper and deeper until you find the big files.

That's the best way in my opinion to "hunt" down big files.

If that does not help you, then maybe you can post the result of "df -h" and "mount" here.