'Your Startup Disk Is Almost Full' on Mac? Do This First

The “Your startup disk is almost full” notification is macOS’s polite way of telling you that disaster is approaching. By the time you see it, you usually have somewhere between 5GB and 15GB of free space — enough that things still work, but not enough for system updates, app installs, or that 4K video you’re trying to import.

Ignore it long enough and weird things start happening. Apps refuse to save. Browsers freeze. Mail stops syncing. macOS starts purging cached files aggressively, which makes everything feel slow. The good news is that you can almost always reclaim 30-100GB in under an hour.

Don’t dismiss the warning

First instinct for most people is to click “Manage…” or just close the notification. Close it if you want, but don’t actually dismiss the underlying problem — once free space drops below about 3GB, macOS starts behaving in genuinely strange ways, and recovering becomes harder.

Apple menu → System Settings → General → Storage. Look at the colored bar. The colors and what they mean:

• Apps (blue) — your installed applications
• Documents (yellow) — anything in your home folder that isn’t categorized elsewhere
• System Data (teal) — the catch-all (we’ll spend most of our time here)
• macOS — the operating system itself
• iCloud Drive — locally cached iCloud files
• Mail, Photos, Music — self-explanatory

System Data is almost always the problem. We’ll get there.

The fastest 10GB you’ll free today

These three steps usually free 10-30GB on a typical Mac, and they take about five minutes total.

1. Empty the Trash on every drive. Right-click the Trash icon → Empty. If you have external drives plugged in, they each have their own Trash — Cmd+Shift+. (period) in Finder shows hidden files, look for .Trashes at the root of each external.
2. Clear out Downloads. ~/Downloads. Sort by Size, delete anything you don’t recognize or actively need. Most of it is one-time-use installers and screenshots.
3. Delete iOS device backups. ~/Library/Application Support/MobileSync/Backup/. Each subfolder is a separate device backup — often 50GB+ each. Keep your current device’s backup if you want; delete the rest.

That alone usually clears the warning.

There’s a faster way to reclaim that spaceSweep handles this automatically and lets you approve before anything’s deleted. Try Sweep free →

Use the built-in Recommendations panel

In the same Storage screen, Apple shows you a few “Recommendations” — Store in iCloud, Optimize Storage, Empty Trash Automatically, Reduce Clutter. These do help, with caveats:

• Store in iCloud moves rarely-used files to iCloud and replaces them locally with stubs. Good if you have iCloud space and decent internet. Bad if your connection is flaky or you work offline often.
• Optimize Storage removes downloaded Apple TV movies and old email attachments after you’ve watched/read them. Genuinely helpful, low risk.
• Empty Trash Automatically deletes Trash contents after 30 days. Mostly safe — just remember nothing in Trash is recoverable after that.
• Reduce Clutter opens a file browser sorted by size. Useful but doesn’t actually reduce anything; it just helps you find big files.

Turn on Empty Trash Automatically and Optimize Storage. The other two are personal calls.

Force Time Machine to thin its snapshots

Even with no backup drive plugged in, macOS keeps APFS snapshots on your internal disk as a Time Machine buffer. They’re invisible. They count toward used space. They might be your single biggest source of System Data bloat.

Open Terminal:

tmutil listlocalsnapshots /

If you see a long list, you’ve got snapshots. Force aggressive thinning:

sudo tmutil thinlocalsnapshots / 9999999999 4

This typically frees 5-30GB instantly. The command tells macOS to thin until it has 9.99 quintillion bytes of free space, which is impossible — meaning it deletes everything it can.

Hunt down the giant files

Spotlight is your friend here. Open any Finder window, hit Cmd+F, then:

• Where: This Mac
• Click “Kind” in the criteria → change to “File Size”
• Set “is greater than” → “1 GB”

You’ll get a sortable list of every file over 1GB on your system. Common culprits:

• Old video projects in Movies
• Virtual machine disk images (.vmdk, .qcow2, .vdi) from Parallels, VMware, VirtualBox
• DMG installers in Downloads you never deleted
• Database files from apps like Photos, Mail, or Notes
• Old GarageBand projects with raw audio takes
• Chrome’s profile folder if you’ve been using it for years
• Random “untitled.mov” exports from QuickTime that were 3GB and forgotten

For each, decide: keep, delete, or move to external. Most large files don’t need to live on your boot drive.

Apps you don’t use anymore

Open Finder → Applications. Sort by Size. Look at the apps over 500MB that you haven’t opened in months. Common bloat:

• Xcode (12-15GB just for the app itself, more for caches)
• Adobe Creative Cloud apps (each is multiple GB)
• Office apps you never use (Word, Excel, PowerPoint individually around 2GB)
• Video editors you tried once
• Old games

Drag them to the Trash. But: dragging an app to Trash leaves all its support files in ~/Library/Application Support, ~/Library/Caches, and ~/Library/Containers — sometimes 5-10x the size of the app itself.

To clean leftovers manually:

1. Note the app’s bundle identifier (right-click → Show Package Contents → Info.plist → CFBundleIdentifier) before dragging
2. After deleting the app, search those Library folders for the bundle ID
3. Delete matching folders/files

It’s tedious for one app. For ten apps, it’s an afternoon. Sweep’s app uninstaller does this whole thing in a single approval per app, but doing it manually is fine if you only have a few to handle.

Don’t dig through ~/Library yourselfSweep hunts down every leftover file an uninstaller misses. Free download for Mac →

Clear the heaviest caches by hand

If you’ve done all of the above and still aren’t where you want to be, hit the heavy-hitter caches directly. Quit each app first.

• Spotify — ~/Library/Caches/com.spotify.client/Data/ (often 4-8GB)
• Slack — ~/Library/Application Support/Slack/Cache/ (often 1-3GB)
• Microsoft Teams — ~/Library/Application Support/Microsoft/Teams/ (multiple subfolders, several GB)
• Adobe — ~/Library/Caches/Adobe/ and ~/Library/Application Support/Adobe/Common/ (often 5-15GB)
• Discord — ~/Library/Application Support/discord/Cache/
• Chrome — ~/Library/Caches/Google/Chrome/
• Mail — ~/Library/Mail/V10/MailData/ (the attachment cache, often 10-20GB)

For Mail specifically, opening Mail and choosing Mailbox → Rebuild on each mailbox is cleaner than just deleting MailData — it preserves your local-only mail.

Check for Photos library bloat

Photos → Settings → iCloud. If “Download Originals to this Mac” is enabled, your entire iCloud Photos library is local — which on a heavy iPhone user is 100GB+.

Switch to “Optimize Mac Storage.” macOS keeps thumbnails locally and pulls full resolution only when you open a photo. You won’t notice the difference for browsing. The space savings are immediate but gradual — macOS thins the local copies over the next few hours.

After all that, restart

Some space won’t actually free up until macOS clears working files. Restart your Mac. Often you’ll see another 1-3GB free up after the boot. Plus everything just runs smoother for a few hours after a clean reboot.

If you’ve followed this whole guide and you’re still in the warning zone, you’ve probably got something specific — a virtual machine, a large database, or a runaway log file. Spotlight search by file size should find it. If you can’t find it, the storage screen in System Settings has gotten more useful in Sonoma 14 and Sequoia 15: click each category’s “i” button and sort by size from there.

The “startup disk almost full” warning isn’t a permanent state. It’s a maintenance task macOS doesn’t do for you. Handle it once thoroughly, set up Empty Trash Automatically, and you can usually go six to twelve months before seeing it again.