The quickest check is the Apple menu: choose System Settings, then General, then Storage. That panel groups files into categories and shows how much each one takes. It is a useful first look, but it is not a complete measurement. The Storage panel rounds figures, hides some system files, and can take a while to refresh. For a second opinion, open Terminal and run `df -h` to see the free space reported by the file system, then `du -sh *` inside a folder to see which items are large. A third option is a disk analyser that scans the whole volume and presents a visual map. The three methods rarely agree exactly, and that disagreement is normal: each one counts different things. The practical rule is to use the Storage panel for orientation, Terminal for confirmation, and a dedicated tool when you need to find individual large files.
If the numbers still do not add up, the gap often sits in hidden folders, local snapshots, or the System Data category. A guide such as Headroom’s Mac storage guides walks through those cases in detail, including how to interpret the categories and where silent filling comes from. The important point is not to trust a single reading. Measure twice, with two different tools, before you delete anything.
What is the System Data category, and what can be removed from it safely?
System Data is a catch-all category in the Storage panel. It collects caches, logs, temporary files, local Time Machine snapshots, and other items that macOS does not classify elsewhere. It is not a single folder you can open and empty. Its size can grow to tens of gigabytes and then shrink on its own after a restart or a backup. Because it is a category, not a location, there is no safe way to delete it as a whole.
What you can remove safely depends on the item inside it. User caches in `~/Library/Caches` can usually be deleted; apps rebuild them, though some will run more slowly on the next launch. Log files in `~/Library/Logs` are safe to clear. Local Time Machine snapshots are safe to remove if you have a recent backup on an external drive; without one, removing them reduces your ability to restore earlier versions. What you should not do is delete files inside `/System`, `/Library`, or any folder whose purpose you cannot identify. Those hold components that macOS needs.
A safer approach is to let macOS manage System Data itself. Restart the Mac, connect the external backup drive and run a backup, then check the Storage panel again. In many cases the category shrinks without any manual deletion. If it does not, inspect the largest items inside it with a disk analyser before touching anything.
In what order should a full disk be cleaned so that nothing needed is lost?
The order matters because some steps create space without risk, while others can remove data you will want later. Start with the reversible steps.
First, empty the Trash and remove large downloads you have already installed or watched. Second, clear browser caches and old iOS backups stored on the Mac, since those are usually duplicated elsewhere. Third, review your photo and video libraries: optimise them to iCloud if you use it, or move older projects to an external drive. Fourth, check for local Time Machine snapshots and remove only the oldest ones if you have a current backup. Fifth, use a disk analyser to find the largest remaining files and decide on each one individually.
At every step, ask whether the file exists somewhere else. If it does, deleting the local copy is low risk. If it does not, move it rather than delete it. Never run a cleaning script or a third-party cleaner that you have not inspected, and never delete a folder you cannot name. The goal is not to free the maximum number of gigabytes in one pass; it is to free space without losing a file you will need next week.
Which tools help find large files and folders?
Terminal is already installed and costs nothing. `du -sh ` lists the size of each item in the current folder; `du -sh | sort -h` sorts them from smallest to largest. `find . -type f -size +1G` locates files larger than one gigabyte. These commands are precise but slow on a full disk and require care: a typo in a delete command can remove the wrong file.
Graphical disk analysers scan the volume and show a treemap or a list of the largest items. They are faster to read and less error-prone for most users. Some are free, some are paid, and the paid ones are not automatically better. A common pattern is to use a free analyser to locate the large files, then decide manually what to do with each one. Cleaning apps that promise automatic removal are a different category: they can help with caches and logs, but they cannot judge whether a file matters to you. Treat their suggestions as candidates, not decisions.
When should libraries move to an external drive?
Moving a library, such as a photo library or a music library, to an external drive frees space on the internal disk and keeps the files accessible. The safe method is to copy the library to the external drive first, verify that it opens correctly, then delete the internal copy. Do not move a library while an app is using it. For photos, you can also use the optimised storage option, which keeps smaller versions on the Mac and full-resolution originals in iCloud. That option depends on a reliable internet connection and on having enough iCloud storage. If either is missing, an external drive is the more predictable choice.
What causes silent filling, and how do you catch it early?
Silent filling is the slow growth of used space without any obvious new files. Common causes include local snapshots, growing caches, log files, and applications that store data in hidden folders. The Storage panel may not update immediately, so the change can appear suddenly. A monthly check is enough for most users: open the Storage panel, run `df -h`, and compare the figures with the previous month. If the free space has dropped by several gigabytes without a clear reason, inspect the System Data category and the largest folders in your home directory. Catching the trend early is easier than cleaning up after the disk is full.