SSD reporting incorrect amount of free space, not due to local snapshots

Question

I recently created a file of just over 20GB and then deleted it shortly after, however my free space did not increase at all. I am using a 2012 MacBook Air with a 128GB SSD and I've had problems before with inaccurate free disk space (even when accounting for Time Machine's Local Snapshots). Usually though I can fix the problem by verifying the disk in disk utility, which then shows Invalid volume free block count and orphaned blocks errors, which I could then repair in recovery mode, but now disk utility repeatedly insists the volume is OK, and its S.M.A.R.T. status is verified.

I suppose I will have to just reformat the drive, reinstall OS X and restore from my backups, but is there anything else I can try first? Also, since this has happened a few times before (mainly when I delete a large amount of space in one go), should I suspect the SSD of having problems, in spite of its verified S.M.A.R.T. status?

UPDATE: I initially thought there was a 26.2 GB discrepancy. Interestingly though, whilst System Information reports that backups are taking up 12.65 GB, sudo du -csh /.MobileBackups gives 31 GB. In total, this adds up to 94 GB of used space, which still amounts to a 7 GB discrepancy which I can't account for.

Answer 1

Ok, so after posting this question I had some major breakthroughs, most of which are posted in the update to my question. By then remembering that du still reports in units of 1024 whilst OS X now uses 1000, the final 7 GB was accounted for (well, there's still a potential 500 MB missing judging by Disk Utility, but that's probably just an artefact of du counting in blocks). Now the only question left is why System Information is reporting a substantially lower value for Backups than it should be, and whether or not it will remember to eventually clean up the other 18 GB of local snapshots.

Answer 2

Try using Grand Perspective to see exactly what's on your disk.

http://grandperspectiv.sourceforge.net/