I was beginning to run low on hard drive space the other day (I was down to ~50 GB on a 480 GB SSD), so I shifted about ~50 GB of data off to another drive. After moving it and running a backup of the external drive, I deleted the data from my internal disk.
I was watching iStat Mini in Notification Center, and I was surprised when I didn’t see my disk use drop by 10%:
I pulled up both the Finder and Disk Utility to see what they were reporting for free space. Finder reported that I had just over 100 GB available, which is what I expected. iStat Mini and Disk Utility were both reporting just about 50 GB available.
I started to panic, worried that something terrible had happened with the drive (John Siracusa has me terrified of HFS+). After the panic subsided, I remembered to check the Storage pane of the About My Mac tool:
And that was the clue: the OS was being too smart for me, holding onto 54.43 GB of data I had deleted as a local backup. Once I connected to my Time Machine disk for a backup, the OS cleaned that up and Finder, iStat Mini and Disk Utility all agreed that I had ~100 GB of free space. I still ran Verify Disk to make sure nothing terrible had happened… and all was ok.
Bottom line: OS X is pretty smart about backups, but Finder obscures this a bit for you. The Storage pane in About My Mac will show you how much of your drive is currently consumed by local backups.