Storage occupied by System Data.....Old trick to free disk space on mac

If a big chunk of your mac storage is taken up by System Data, then use the old trick of forcing macOS to clear up much of the non-essential data like this:

1. Backup your files and data (Time Machine is recommended).
2. Fill you mac with files so you only have about 20GB of free disk space left.
3. Now open Appstore and search for the latest macOS version (or atleast the one currently installed on your system if you don't want to upgrade).
4. Download it (even if it is the same one currently on your system or even if you are on a lower version and you don't wish to upgrade) - if nothing happens, press download again. If mac gives low disk space error, free up some space (not more than 5 GB). The idea is to have just enough free space for the download to start.
5. macOS will now begin to free up space by clearing old system snapshots, temp files, cache, logs and other non-essential data.
6. If the download completes, you'll be asked to agree to Apple T&C. You can safely disagree and quit the installer, or continue if you want to upgrade/ reinstall to fix any existing problems.
7. Reboot to safe mode.
8. Reboot normally.

This should help you regain a substantial amount of free space that is occupied by System Data that users cannot release on their own.

As far as I know, mine is still occupying a lot of space, but it is much lesser than earlier when System Data was over 75 GB.

EVEN BETTER METHOD: REQUIRES TIME AND PATIENCE

1. Backup all your files and data (Time Machine is recommended).
2. Reboot to recovery.
3. Install a fresh copy of macOS (either from internet or from an installer). This will erase your boot disk and do a clean install.
4. Reboot.
5. Restore the Time Machine backup.
6. You may have to login to some of your cloud accounts.
7. You have to give some time for the system and apps to reindex, regenerate cache, etc to perform optimally.

Hope this helps.

[Edited by Moderator]