I have used DiskUsage App. But in the grid i get system data using around 8GB out of the 24GB usable memory of my Phone[32GB internal] I have used DiskUsage App. But in the grid i get system data using around 8GB out of the 24GB usable memory of my Phone[32GB internal]

Application data also is mentioned seperate. I can't figure out by what 8GB is being used. I have 32GB internal memory out of which OS has already utilized 8GB and now another 8GB is being used as System Data which does not include Applications installed and it's data. Please suggest? Not able to see what is occupied in that 8GB?
Just going to chime in here that I also had this problem - "System Data" segment was consuming about 7.5GB of space on my 16GB Moto X. Often this can be caused by a rogue app that isn't cleaning up after itself.  I deleted almost everything extraneous I could think of and finally hit upon the solution (for this case, anyway).

In this case the culprit happened to be the default android Mail app when sync'd to an Exchange account. I was set up to sync only mail from the previous week, and indeed if you scrolled through the inbox only about a week of mail was there, but on a hunch I deleted the Exchange account and then re-made it. It synced up about a week of mail and otherwise seemed exactly the same as it was before but it freed up almost 5GB of storage space in System Data. 

This is Android 6.0 - go figure that a core application would leak so badly.