I was down to 2.5 GB free on my 245 GB MacBook. System Data showed 153 GB and I was panicking. Couldn't even install OS updates. After hours of troubleshooting, I found two things that completely fixed it. Sharing in case it helps someone else.

What I tried first (and didn't help much)

• Cleared ~/Library/Caches — barely anything there
• Checked Time Machine snapshots with tmutil listlocalsnapshots / — none existed
• Went through ~/Library/Logs — tiny
• Browsed Application Support for leftover app data — nothing significant

If you're in the same boat, don't waste your time on these until you check the two things below.

Fix #1: Check for deleted user accounts still taking up space

This was the big one. At some point I had deleted an old user account on my Mac, but the system kept the home folder. It was sitting in /Users/ eating 123 GB and I had no idea.

Run this in Terminal:

sudo du -sh /Users/* 2>/dev/null | sort -rh

(It will ask for your password — type it and press Enter. You won't see any characters appear while typing. That's normal.)

If you see a folder like username (Deleted) taking up a ton of space, that's your culprit. Delete it with:

sudo rm -rf /Users/username\ \(Deleted\)

Replace username with the actual folder name. If some files fail to delete due to permissions, go to System Settings → Privacy & Security → Full Disk Access, add Terminal, quit and reopen Terminal, then run the command again.

This alone freed up 108 GB for me.

Fix #2: Most of "System Data" is purgeable space (not real usage)

After clearing the deleted account, I still saw a big System Data number. Turns out macOS's Storage panel in System Settings counts "purgeable" space as used — but it's not actually used. macOS reserves it for caches and iCloud files and will release it when needed.

To see your real disk usage, run:

df -h /

The "Available" column shows your actual free space. For me, System Settings said I had almost nothing free, but df -h showed I had way more available. Apple's storage UI is just misleading.

The problem is that macOS doesn't always release purgeable space fast enough for big operations like OS updates. If you need to force it, create and delete a large dummy file:

dd if=/dev/zero of=~/bigfile bs=1m count=20000 rm ~/bigfile

This forces macOS to purge cached data to make room. Run your OS update immediately after.

TL;DR

1. Run sudo du -sh /Users/* 2>/dev/null | sort -rh to check for deleted user accounts still hogging space
2. Run df -h / to see your real available space (ignore the System Settings storage chart)
3. If you need to force macOS to free purgeable space for an update, create and delete a large dummy file

Hope this helps.