You should aim to have 15-20% of your total internal storage as free space (~20-25 GB as an absolute minimum). If you do not have enough free space on your Mac’s internal storage, then here are some steps to take to safely free up space:
Free up storage space on Mac - Apple Support
Optimize storage space on your Mac - Apple Support
How to free up ‘System Data’ and other st… - Apple Community
"System Data" comprises your data, but they are typically hidden files that Spotlight cannot assign to a specific category like Documents or Applications. Examples include Outlook attachments, iOS updates, Cloud storage caches, etc. Starting with macOS 27 Golden Gate, System Data also includes what macOS used to call ‘purgeable’ storage – system caches, Time Machine snapshots and some other data, though the user had no control over the purging. That means macOS 27 makes System Data bigger, not smaller.
Trying to manually clean out hidden files in System Data is likely to cause more problems than it solves, macOS hides those files for a good reason. Follow the steps in the linked articles above.
On macOS 27, separate ‘purgeable’ space is no longer reported, and instead those data are now considered as used space. They are also now included in System Data, accounting for the apparent increase you would see. You won't lose space, macOS just categorizes things differently than on Tahoe.
More detail on this change in this User Tip:
System Data and Free Space Changes in Gol… - Apple Community
The best advice is to just ignore the System Data category. ‘System Data’ is merely a catch-all category for files that Spotlight cannot assign to specific categories like Applications, Documents, Photos, etc. In fact, what is now shown as System Data used to be reported as ‘Other’, Apple changed the label in macOS 12 Monterey. System Data probably sounds better, but Other was closer to reality.
