Mac lost disk space overnight

Sudden loss of space feels alarming and almost always has a boring explanation. The useful move is not to guess but to ask the file system what changed, which takes two commands.

First, the four usual causes

Cause	How much	How to confirm
A Time Machine snapshot	Several GB to tens of GB	`tmutil listlocalsnapshots /`
A macOS update downloaded overnight	Tens of GB	Look in Applications for an installer
A backup or sync that ran	Varies	Check the device backup folder
Photo or media analysis rebuilding	A few GB	Check container sizes

Then ask what actually changed

find ~ -type f -mtime -1 -size +100M \
  -exec ls -lh {} + 2>/dev/null | sort -k5 -h | tail -20

Every file over 100 MB modified in the last day, largest last. This is the single most useful command for this problem, because it answers the question directly rather than by elimination. Change -mtime -1 to -mtime -7 for the last week.

To include the parts of the disk outside your home folder, run the same search against the data volume with sudo. Expect a lot of system noise there, so keep the size filter high.

Snapshots, which are the most common answer

tmutil listlocalsnapshots /
df -h /System/Volumes/Data

A snapshot is taken hourly when Time Machine is on, and one taken just before you deleted a large file holds onto that file's space. That is why the space you freed last night is gone again this morning, and it is purgeable rather than lost.

Build a baseline so this is easy next time

du -xh -d 2 ~ 2>/dev/null | sort -h | tail -40 > ~/Desktop/space-$(date +%F).txt

Run that occasionally. Two files a week apart turn a vague feeling into a specific folder, and comparing them takes seconds:

diff ~/Desktop/space-2026-09-01.txt ~/Desktop/space-2026-09-09.txt

When it is none of those

If nothing changed in your home folder and there are no snapshots, the remaining candidates are another user account, a virtual machine that expanded, or a log that ran away. Those are covered in old user accounts, virtual machines and system logs respectively.

Common questions

Why did my Mac lose disk space overnight?

Most often a local Time Machine snapshot, an overnight macOS update download, or a device backup. All three are normal and the first releases space automatically when the disk gets tight.

How do I find what changed on my Mac's disk?

Use find with a modification time and size filter, for example files over 100 MB changed in the last day. It answers the question directly, unlike elimination by folder, and it takes seconds to run.

Can I stop macOS taking local snapshots?

Only by turning off Time Machine, which is a poor trade for most people since snapshots are what let you recover a file deleted this morning. They are thinned automatically when free space runs short.

How can I track disk usage over time on a Mac?

Save a du listing to a file occasionally and compare two of them with diff. Two measurements a week apart identify the growing folder immediately, which no single measurement can do.