If you do a web search on macOS System Data, you’ll find easily more than a dozen articles that discuss this topic, and how Apple has used a succession of sometimes similar, sometimes disparate terms to describe what is currently listed as “System Data” (and ways to reduce it if it’s taking up “too much” space).

The problem is that System Data includes lots of tiny files buried in “Library” Folders, as well as Time Machine Snapshots, iOS and iPadOS backups to your Mac, and others.

Many of these articles discuss “System Data” as excessive when it exceeds 40-50 GB, but almost 90% of ALL the space on a 1 TB internal SSD drive is just enormous.

Unless you have AppleCare, your Mac is likely out of warranty, but if you describe your problem to Apple on its support page, you might get lucky and be able to talk live to a support engineer.

Here’s one of the articles I found on the topic:

Some of these articles suggest employing third party “cleanup” Apps, some of them not frequently discussed on listservs such as this, and I’d be VERY careful about resorting ?to one of those without a very clear understanding of what you’re doing.

Potential candidates for what’s bloating your “System Storage” might include doing Time Machine Backups directly onto your boot volume (not just the snapshots), huge attachments to messages, emails, links to previously watched movies. But there is something seriously wrong with what’s being stored on your Mac.

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Jim Robertson