iPhone System Data taking up 30GB

Before going as far as erase, check Settings > General > iPhone Storage and scroll the app list. If there's an iOS update sitting there downloaded but not installed, delete it, those are a few GB each and sometimes get counted in System Data. Also give that screen a minute to finish calculating. The number on first open is often wrong and settles lower.

System Data is mostly caches, logs and temporary files, and you can't delete it directly, which is the annoying part. Things that do shrink it: - Restart the phone, then leave it on charge on wifi overnight. iOS clears a lot of that cache on its own when it's idle. - Settings > Safari > Clear History and Website Data. - Big message attachments and streaming app caches (Netflix, Spotify, podcasts downloads) sometimes get counted there. Clearing downloads inside those apps helps. - Installing a pending iOS update often drops it noticeably. If none of that touches it, back up to iCloud or a computer, erase, and restore from the backup. That's the only guaranteed fix, and after the restore System Data is usually back to a few GB.