Recover missing disk space on OSX

I had a lot of trouble on my Mac with Time Machine telling me the disk was full and yet I could not find huge wins just by inspection and deleting things that looked too big. I drafted ChatGPT in to help diagnose the problem and it had me jump through all sorts of hoops, which did eventually recover the space I needed.

Rather than repeating all those steps as an aide-memoire , I asked ChatGPT to go through the entire exchange and prioritise the various steps. This (lightly edited) is what I received.

Retrospective: Better Process for Identifying the Missing Disk Space

Yes. In hindsight, once we saw the largedf vs du discrepancy, rebooting should have moved much higher in the priority list.

A better priority order would have been:

1. Confirm real free space withdf

   df -h /System/Volumes/Data
   

2. Compare with visible file usage

   sudo du -xhd 1 /System/Volumes/Data 2>/dev/null | sort -h
   

3. Ifdf says ~203 GB used but du only finds ~119 GB, stop hunting normal folders

That discrepancy was the crucial clue. At that point, deleting caches, MobileSync backups, and Homebrew files was secondary.

1. Check for snapshots and open deleted files

   sudo diskutil apfs listSnapshots /System/Volumes/Data sudo lsof +L1
   

2. If snapshots are mounted/protected/busy, reboot early

The moment we saw: /Volumes/com.apple.TimeMachine.localsnapshots/... and later Snapshot ... Purgeable: Yes a reboot should have been recommended as the quickest practical next step.

So yes: once the evidence showed ~80--100 GB missing from normal filesystem accounting , the best path was:

df/du discrepancy → check snapshots/open files → reboot → recheck df

rather than continuing to inspect ordinary folders.

The cache/MobileSync/Homebrew cleanup advice was still safe and useful, but it was not the main solution. The decisive clue was the df/du gap, and the decisive fix was rebooting to release mounted Time Machine local snapshots.

I'm sure this will help future me. May it also help present you.