tl;dr - you might want to check for offloaded apps. Those might count towards a high System Data usage as well. Just adding to the discussion since I recently discovered that offloaded app storage counts towards the System Data usage. It might have been mentioned here, but I just skimmed throught the responses... In my case I had ~65Gb System Data usage, but when re-downloading one specific video app it became evident that 50Gb of those were from that app. So after re-downloading, System Data usage was reduced by 50Gb - but of course that was just moved to "Applications". But now it's at least evident what app is using those 50Gb:s, giving me a fair chance to clean it up.