Mac Storage Full But Nothing There? Where It Went

The most frustrating version of a full disk is the one where you delete things and the number does not move, and Storage says 60 GB of "System Data" that you cannot open. That category is real, it is explainable, and most of it can be recovered.

Get honest numbers first

The Storage screen in System Settings is a summary and it lags. Two commands give you the truth:

# actual free space on the boot volume
df -h /

# biggest folders in your home directory
du -sh ~/* 2>/dev/null | sort -h | tail -15

# drill into whichever one is large
du -sh ~/Library/* 2>/dev/null | sort -h | tail -15

The second command is the one that ends most investigations. Common surprises: ~/Library/Developer (Xcode simulators and archives, tens of gigabytes), ~/Library/Containers, ~/Library/Application Support, ~/Library/Mobile Documents, and Downloads folders nobody has opened in three years.

What "System Data" actually is

It is macOS's catch-all for anything it cannot file under Apps, Documents, Photos and so on. In practice it is:

• Time Machine local snapshots — usually the largest single item.
• Caches — system and app caches, which macOS purges when it needs space.
• Virtual machine images and simulators.
• Mail attachments and message archives.
• Log files, sleep image and swap files.

Local snapshots: the big one

Time Machine keeps hourly snapshots on your internal disk even when your backup drive is elsewhere. macOS reports this space as available, in the sense that it will delete snapshots when you need room, but the Storage screen counts it as used, which is why the numbers appear to contradict each other.

# list snapshots stored on the disk
tmutil listlocalsnapshots /

# how much space snapshots are actually using
sudo tmutil listlocalsnapshotdates /

You usually do not need to delete these by hand — free up real space and macOS reclaims them automatically. If you must, thin them with tmutil thinlocalsnapshots / 10000000000 4, which asks macOS to free roughly 10 GB. Deleting all snapshots removes your ability to restore recent versions until the next full backup, so treat it as a last resort rather than routine maintenance.

The places to check next

1. Trash on every volume. External drives have their own Trash and it does not empty when you empty the main one.
2. Downloads. Sort by size in Finder list view. Old installers and disk images are usually the top of the list.
3. Mail. Downloaded attachments live in ~/Library/Containers/com.apple.mail and can run to many gigabytes.
4. iOS backups. In ~/Library/Application Support/MobileSync/Backup, and often several versions of the same phone.
5. Photos. If iCloud Photos is on, turn on Optimise Mac Storage in Photos settings and originals move to the cloud.
6. Developer folders. Xcode's derived data, archives and simulator runtimes, all in ~/Library/Developer.

Purgeable space, and why the numbers disagree

macOS reports some space as "purgeable": files it can delete on demand, including snapshots, cached iCloud files and optimised media. Finder counts it as available, System Settings shows it as used, and some third-party tools ignore it entirely. That is the whole reason three tools give you three different numbers.

# Finder's view, which includes purgeable space
df -h /

# the stricter view, which does not
diskutil info / | grep -E "Free Space|Container Free Space"

The practical effect: if you are simply trying to install something large, macOS will usually free what it needs on demand and the installation succeeds despite the alarming number. If a copy actually fails for space, that is when it is time to delete real files.

What not to do

Do not delete files from /System, /private/var or anything you cannot identify, and be sceptical of tools promising to clean gigabytes of junk. Caches are rebuilt, usually more slowly than they were deleted, and the space returns within days. The durable wins are always the same four: snapshots, downloads, media libraries and developer folders.