I have an USB3 drive I use for backups. It has two folders, one is 80GB and one is 450GB. It's a 1TB drive with 150GB free. I don't have a Ph.D in mathematics, but I'm pretty sure that's not quite correct. Ran First Aid, all good. Disk Utility still shows 150GBs free. I used the command-option-period trick and there are a bunch if hidden folders for spotlight indexing, trash etc. There is perhaps 2GB of storage space being used by Spotlight (but of a surprise) but nothing that would account for any significant amount of my missing storage. Any ideas? Disk Utility shows it's in good shape but I can't find where the storage is being used.