Thought I’d post this in case anyone else has a similar problem.
Our Head of School’s MacBook Pro was having problems. It was eating up disk space for no apparent reason. It seemed to be happening during the synchronization of the mobile home directory. Running lots of apps including OmniDiskSweeper were reporting that while the disk had 216GB used, it was only cataloguing 50GB.
With the suggestion of a fabulous engineer at Apple, I ran the program as the superuser
sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
and it found hundreds of gigabytes in the user’s desktop directory all called silly names like…
.fstemp.+PHD-L-hDjzWLUASp4 ... 3Z1.noindex
I’m deleting those.