“Your Startup Disk Is Almost Full”
How to Free Space Safely

Your Mac Isn't Broken. It's Out of Elbow Room.

Every Mac has one built-in drive — Apple calls it the startup disk, because it's the drive your Mac starts up from. It holds macOS, your apps, your documents, your photos, and a pile of temporary files the system makes while it works. When that drive gets close to full, macOS puts up the warning. That's all the message means. Nothing is infected, nothing is failing, and you almost certainly don't need to buy anything.

The trouble is what the warning pushes people toward: deleting things they shouldn't, or buying a “Mac cleaner” app with a scary scan and a monthly subscription. Neither is necessary. Apple put every tool you need into System Settings, free. Below is the order I work in on a house call — biggest and safest wins first, and a short list of things to leave completely alone.

Start Here: The One Screen That Tells You Everything

1. Click the Apple menu in the top-left corner of the screen.
2. Choose System Settings.
3. Click General in the sidebar on the left — you may need to scroll down.
4. Click Storage on the right, then wait a few seconds. The colored bar takes a moment to add everything up; that pause is normal, not a freeze.
5. On an older Mac running macOS Monterey 12 or earlier, the path is Apple menu → About This Mac → Storage → Manage instead. Apple moved it into System Settings starting with Ventura 13, and that's where it lives in macOS Tahoe 26.

Quick Win 1: Empty the Trash (Yes, Really)

This one catches nearly everybody. When you drag a file to the Trash, macOS doesn't erase it — it moves it. The space isn't yours again until you empty the Trash. I've opened Trash cans holding 40 gigabytes. Click the Trash icon at the end of the Dock, have one last look, then click Empty in the top-right corner. Big Trash takes a minute or two — let it finish. Then do the two most-forgotten cousins: the Recently Deleted album in Photos (it holds pictures for 30 days) and, in Mail, Mailbox → Erase Junk Mail followed by Mailbox → Erase Deleted Items. All three hold real space, and all three are safe to empty.

Quick Win 2: The Downloads Folder

1. In the Finder, choose Go → Downloads from the menu bar (or click Downloads in the Finder sidebar).
2. Switch to list view, then click the Size column heading to sort biggest-first. Click it twice if the small ones come up first.
3. Delete the obvious: anything ending in .dmg or .pkg — those are app installers, the shipping box the app arrived in. The app itself is already installed elsewhere, and you can re-download the installer free if you ever need it again.
4. Then the duplicates: “statement (3).pdf,” the same photo saved four times, attachments from 2021.
5. Anything you want to keep, drag to Documents or Desktop where you'll actually find it later.
6. Empty the Trash again when you're done — otherwise you've just moved the pile.

Turn On Apple's Own Recommendations

• Optimize Storage — automatically removes Apple TV movies and shows you've already watched, and keeps only recent email attachments on this Mac. Turn it on. Almost no downside.
• Empty Trash Automatically — erases anything that's been in the Trash more than 30 days. Turn it on and forget it. Thirty days is plenty of grace period.
• Store in iCloud — keeps Desktop and Documents in iCloud Drive, photos and videos in iCloud Photos, and messages in iCloud, leaving only recently opened files on the Mac when space is needed. Powerful, but think about it for a minute first: it uses your iCloud storage (everyone gets 5 GB free; paid plans start at 50 GB for about $1 a month), offloaded files need an internet connection to come back, and it is not a backup — delete a file on the Mac and it's gone from iCloud too.
• A fair middle path: turn on Optimize Mac Storage for Photos only (Photos → Settings → iCloud) and leave Desktop and Documents on the Mac. Photos is usually the biggest category anyway.

Hunt the Big Files — macOS Has a Finder for This

Twenty minutes on the big files beats a whole day on the small ones, and the tool is built in. From System Settings → General → Storage, find Documents in the list and click the small info button beside it. Across the top you'll see Large Files, Downloads, and File Browser. Click Large Files and sort by Size — or by Last Accessed to spot what you haven't opened in years. Select something and click Delete, or click Show in Finder first for a proper look. What's usually hiding there: phone videos and screen recordings (the giants), old zip archives, forgotten installers. One rule for this page — if you don't recognize a file, leave it alone. “I don't know what that is” is a reason to skip it, not a reason to delete it.

What “System Data” Really Is (and Why 60 GB Is Normal)

This is the question I get more than any other, and the answer is reassuring. In Apple's own words, System Data “contains files that don't fall into the categories listed here” — log files, caches, virtual-memory files and other runtime system resources, plus temporary files, fonts, app support files and plug-ins. Apple is equally plain about the rest: you can't manage the contents of this category. macOS manages it, and it grows and shrinks depending on what your Mac has been doing. Much of it is “purgeable” — Apple distinguishes free space from available space, and available space includes caches the system throws away the instant something needs the room. The only safe shortcut is a full shut-down and restart, which clears a surprising amount on its own. A big System Data number is also the exact hook every “your Mac is full of junk” advertisement uses. Don't chase it.

What NOT to Delete — the Short, Important List

• The System folder. This is macOS itself. It's protected for good reason, and there's no space to win.
• Any Library folder. Holds the settings, licenses and support files your apps need to run. Deleting the wrong item breaks working software.
• The macOS category in Storage settings. Same story — it has no info button because it isn't yours to trim.
• System Data. Apple states plainly you can't manage it. Let macOS handle it.
• Anything named “cache” or ending in “.plist.” Deleting these by hand is the classic way to turn a small problem into a big one.
• Time Machine local snapshots. macOS creates and removes these automatically; they're already purgeable.
• Any file you don't recognize. Oldest rule in the trade: unknown is not the same as unnecessary.

Skip the Paid “Mac Cleaner” Apps

• The “junk” they count is largely caches and temporary files — the purgeable stuff macOS discards on its own when space is needed.
• The real space comes from Trash, Downloads and large files. You just did all three, free, in about twenty minutes.
• They typically want deep access to your whole drive plus an auto-renewing subscription that's harder to cancel than it was to start.
• A web page shouting “YOUR MAC IS INFECTED — 5 VIRUSES FOUND” with a countdown is lying: a website cannot scan your Mac, ever. And a phone number in a pop-up is never Apple's.
• If one of those pop-ups is on screen now, don't click anything inside it — not even the X. Quit the browser with Command + Q, or press Command + Option + Esc and force quit it. Nothing has happened to your Mac.

When Cleaning Isn't Enough Anymore

If the warning comes back a month later, and again the month after, that's not a failure of housekeeping — it's a drive too small for the life you're living. My first answer is an external drive: a portable 1 TB USB-C SSD runs roughly $70 to $120 as of mid-2026, often less than a year of extra iCloud storage, and it holds photo libraries and video beautifully. Second answer is a bigger iCloud plan if your files need to follow you to an iPhone and iPad too. And here's the part nobody enjoys hearing: on modern Macs the internal drive is not upgradeable — it's soldered to the logic board, so a 256 GB Mac is a 256 GB Mac for life. That's not a reason to replace a machine you like. It is worth knowing before your next purchase, when paying a little more up front for 512 GB or 1 TB saves this whole conversation. I don't sell computers, so take that for exactly what it's worth.

What's in the 27-page PDF

• What the warning actually means — and why free space matters more than people think
• The whole fix in six checkboxes, plus the one mistake that costs the most
• The exact path in macOS Tahoe 26: System Settings → General → Storage (and the older About This Mac route)
• Reading the storage bar: a plain-English table of every category macOS shows you
• The “System Data” mystery explained — why it's huge, why it's fine, and the only safe shortcut
• Quick Win 1: the Trash, Photos' Recently Deleted, and Mail's deleted items
• Quick Win 2: the Downloads folder, sorted by size — usually the easiest 5–20 GB in the house
• Apple's three Recommendations compared: Store in iCloud, Optimize Storage, Empty Trash Automatically
• “Store in iCloud,” honestly — what it's good at, and the four things to know before switching it on
• Finding the big files: Documents → Large Files, plus photos, videos, Mail, Messages and old iPhone backups
• The short list of what to never delete, and why I don't recommend any paid Mac cleaner
• The long-term fix, a refrigerator checklist, kitchen-table Q&A, a glossary, and a fill-in cheat sheet

Why I wrote this

“Your startup disk is almost full” is one of the most common messages I get called about, and it's one of the least dangerous — but it's dressed up to look urgent, which is exactly when people delete something they shouldn't or hand a credit card to a cleaner app. Everything that actually works is free and already on the Mac. I'd rather you have the order written down than pay rent on it. I write these guides as printable handouts so the checklist can live next to the computer, and if you'd rather do it with company, that's a flat $99 visit with a 30-day follow-up included. — Bill