System Data, technically, has a mount point /System/Volume/Data
With a mount point of /System , this Volume is part of the Operating System and can not be deleted - full stop
It is where your User Account (s) was made when the computer was First Setup as New
We can point-out what may be included in this Volume
Though, how to reduce this is a long and complicated trial and error process
The category is a collection of files which include:
• System temporary files
• macOS system folders
• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)
• Personal user data
• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)
• Cache files
• Fonts, plugins, extensions
• Other files that are not recognized by a Spotlight search
• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package
•Time Machine Backup Snapshots that have not been transferred to the TM Backup Drive
Was there or is there another user account currently or previous used on this computer ?
Has the account been removed ?
Did the removed user account ’s Home Folder also been removed ?
Delete a user or group on Mac
They can be located anywhere on your hard drive. The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders. You can use either of these two free apps, GrandPerspective , to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.
