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 large df vs du discrepancy, rebooting should have moved much higher in the priority list. A better priority order would have been: Confirm real free space with df df -h /System/Volumes/Data Compare with visible file usage sudo du -xhd 1 /System/Volumes/Data 2>/dev/null | sort -h If df says \~203 GB used but du only finds \~119 GB, stop hunting normal folders That discrepancy was the…