I have bought Lenovo k4 note n have recently upgraded to marshmallow. So I ve 16 GB of internal memory out of which the Android 6.0 inbuilt memory app inside storage and USB setting, says 5.37 GB is OS occupied. 9.46 GB out of 10.63 GB is used. Further break down are as follows: Apps 5.29 GB Images 105 MB Videos 8 KB Audio 588 KB Other 488 MB Cached Data 183 MB Which totals to 6.066 GB. So now subtracting 9.46 with 6.066 GB I get 3.394 GB which I can't understand where it went. Still after going inside file manager n clicking the view hidden files these are what I get: Android 2.17 GB Download 278.01 MB Shareit 200 MB WhatsApp 136 MB Pictures 74 MB Books 52 MB DCIM 29 MB Which comes to around 3 something GB and still 3 GB are missing.. so.. around 6 GB of files are not showing.. I am unable to find the files that are taking up space. Can anyone help please.
I also had the same problem with my Android.
But after doing some researches i found that the google apps Google docs Google sheets and Google slide are taking much space .
When i looked for the app info of these apps, i found that all of these are taking at least 200mb of space where as i never use them.
So i cleared the data and all problems solved.
But if still the problem does not solve, you should finally factory reset your device.
Hope it helps.
There might be some hidden files on your phone that occupy the 3 GB download ES file explorer . Enable show hidden files and llok for the files, hopefully you'll find that 3 GB without much digging