Missing storage space due to 2gb 'system data' - how to remove? I'm hoping someone can help.

My 32GB Nexus 9 tablet shows 21.64GB of available storage space out of total space of 25.61GB (3.97 used in total). The used space only adds up to 1.97GB.  The diskusage app reports the remaining 2gb as 'system data' but doesnt show the details.

Strangely, My 32GB Nexus 6 phone reports the identical apps and data correctly - 24.07GB available out of 25.98 total, with only minimal 'system data'.

Does anyone know what this 'system data' is and how to remove it on the Nexus 9?  A hard reset didn't help.

Many thanks
"System data" is... Well, the data that you need for your system to run. They are the files in your /system directory. You need those files there, otherwise you'd have a brick; your phone would be unusable without an operating system. A factory reset does not remove it because it only removes all user data (all files stored in /data directory.

Also, you may be confused as to why the total storage doesn't add up to 32GB. Hardware manufacturers refer to a GB as 1000 MB, which is 1000 KB, which is 1000 B. So there is a disparity here that gets bigger, the higher the number.