Your drive says it's full. The built-in storage overview shows a coloured bar with a large segment labelled "Other" or "System Data," and no way to click into it.
Disk analyzers solve this. They scan every folder, add up what's inside, and show you the actual answer — usually in a visual map where the size of each block corresponds to the size of the folder.
They're the right first step before any storage cleanup, because they replace guessing with looking.
How they work
An analyzer walks your entire folder tree, recording the size of every file, and totals them up per folder. What you get is a view your file manager can't give you: folder sizes, sorted, all the way down.
That's the whole trick. Your file manager shows file sizes but generally not folder sizes without asking one at a time. The analyzer computes them all.
The first scan takes several minutes to half an hour depending on the drive. Subsequent scans are faster.
Reading a treemap
Most analyzers display results as a treemap: a rectangle divided into smaller rectangles, where area equals size. One block taking up a quarter of the screen is a quarter of your drive.
Colours usually indicate file type — video, images, documents, application data. Clicking a block drills into it.
What to look for:
One enormous block. The most common finding, and usually something you'd forgotten: a virtual machine image, a video project, a photo library, a cache folder.
Many similar medium blocks. A pattern rather than a single culprit — years of exports, a duplicated library, a folder of installers.
A large block you don't recognize. Investigate before deleting. It's often an application's support folder, which may contain your actual data.
Deeply nested surprises. Analyzers reveal things buried four levels down that you'd never have found by browsing.
Some tools present the same data as a sorted list rather than a map. Both work; the list is easier to read precisely, the map is better for spotting patterns at a glance.
Decoding the confusing categories
The built-in storage overviews use categories that raise more questions than they answer.
"System Data" or "Other" — the catch-all. It typically includes caches, logs, temporary files, system snapshots, application support files, and anything the categorizer couldn't identify. On a Mac this can be startlingly large.
The common contents:
- Local snapshots taken by the backup system, held on the internal drive when the backup drive isn't connected. These usually clear themselves as space is needed, but they can occupy a lot in the meantime.
- Caches from browsers and applications.
- Logs, which accumulate slowly and can grow very large if something is repeatedly erroring.
- Application support data, which may include your actual documents for apps that store data in their own format.
- Old installers and update files.
"Applications" being larger than expected — modern applications bundle multiple versions for different processor types, and some include very large resource files.
"Documents" being enormous — this category often catches anything that isn't clearly media or an application, which includes disk images, archives, and virtual machines.
Purgeable or purgeable space — files the system believes it can delete if needed, mostly caches and cloud-optimized files. It counts as used and also as available, which is why the numbers sometimes don't reconcile.
The usual suspects an analyzer reveals
In roughly the order of frequency:
1. Old phone backups stored locally, from devices you no longer own.
2. Virtual machine images. Tens of gigabytes each, easy to forget.
3. Video editing caches and renders. Editing applications keep preview and render files that can exceed the original footage several times over.
4. Duplicate photo libraries. Very common after migrating between applications, or importing the same source twice.
5. Downloads, spanning years.
6. Development dependency folders. If anyone in the household writes code, these are numerous and enormous.
7. Game installations, particularly ones nobody has played in two years.
8. Cloud sync folders set to keep everything local, when on-demand mode would use almost nothing.
9. Mail attachments, if you use a desktop email client that downloads everything.
10. Log files from an application that's been erroring quietly for months.
What to do with what you find
Before deleting anything large: understand what it is. Search the folder name if you don't recognize it. Applications store real data in places with unhelpful names.
Safe to delete: downloads, caches, old installers, obviously duplicated material, old backups of devices you don't own.
Move, don't delete: media libraries, finished projects, archives. Copy to an external drive, verify by opening files, then remove.
Configure rather than delete: cloud sync set to on-demand, photo library set to optimize storage, editing application caches capped or relocated to an external drive.
Leave alone: system folders, application support folders you don't understand, anything inside a photo library package.
Choosing a tool
The category is mature and the options are broadly equivalent. What to look for:
Free versions exist and are adequate. Several well-established free analyzers do everything most people need. There's no requirement to pay.
It should show a visual map or a sorted tree, with drill-down.
It should be able to scan external drives, which is useful for auditing an archive drive.
Be cautious with tools that bundle "cleaning" features. Analyzers that show you what's there are safe. Automated cleaners that delete based on their own criteria are a different category, and some are aggressive, some are unnecessary, and a few are outright scams. Use the analyzer to look; do the deleting yourself.
Avoid anything advertised through pop-ups or promising dramatic speed improvements. That corner of the market is not reputable.
Check whether it needs full disk access. Most do, in order to scan everything. That's a legitimate requirement and also a reason to use an established tool rather than the first search result.
Auditing an external drive
Analyzers are also useful pointed at drives that aren't your main one.
Run one against your archive drive and you'll typically find: duplicated project folders, several copies of the same media at different qualities, backups of backups, and material you have elsewhere.
This is worth doing before buying a larger drive. Most archive drives are 20–40% redundant.
The habit worth building
Run an analyzer twice a year, whether or not you're short on space.
It takes ten minutes and it catches trends — the folder that's been growing quietly, the cache that's got out of hand, the application that's writing more than you realized. Finding these while there's still headroom is much easier than finding them at 98% full with a document that won't save.
Note what you find in a file, with dates. Comparing this year's scan to last year's tells you where your data is actually growing, which is the information you need when deciding whether to buy a bigger drive, a cloud plan, or a NAS.
Next in this series: Internal vs External vs Network Storage: A Beginner's Guide