A large System Data category is not a list of files that can all be deleted. Apple uses it for material that does not fit a more specific storage category, including both Apple and third-party files.

Start with files you can identify

In macOS Ventura 13 or later, open System Settings, General, Storage. Earlier releases show storage through About This Mac. Let the information populate, then inspect categories that offer a detail or management button.

Look first at recognisable material: old installers in Downloads, media you no longer need locally, applications you no longer use and completed project exports. Check ownership before removing shared files or another user's work.

For each candidate, ask whether it is the only copy, whether it can genuinely be downloaded again and whether an application still depends on it. A large folder with a vague name is a reason to investigate its owner, not a reason to delete it first.

Move deliberately and measure the result

If you need to retain a file, copy it to suitable independent storage and open the destination copy before removing the original. Application libraries need their own documented move procedure; do not pull files out of a Photos library package to make the storage bar smaller.

Review the Trash before emptying it. Apple notes that moving an item to the Trash does not release its space until the Trash is emptied. Keep recovery needs in mind before taking that final step.

Return to Storage after each meaningful change and see whether enough room is available for the actual task, such as an update or file copy. Avoid adopting a fixed percentage as a universal technical requirement.

If the unexplained category remains large, record the applications in use and recent changes. Preserve the data and investigate those applications' storage controls. Repeatedly deleting unfamiliar Library or system folders can create a second problem without explaining the first.

Source: Apple's Mac storage guidance.

For help with your Mac, contact AEPC in East Auckland or call 027 908 8880 to discuss your requirements.