As people have been upgrading their Macs to macOS Sierra, I’ve heard a number of people say that they’ve got 10, or ever 30 GB more space than before. When they look in the Finder, they see a certain amount of free space on their startup volumes, and that number is much higher than it was before the upgrade.
Unfortunately, it’s an illusion.
Here’s an example. My MacBook has a 256 SSD, and here’s what I see if I examine its storage:
Note that, on the left, the disk claims to have 150.8 GB free. I’ve hovered over the free space section, and it’s 146.17 GB. If I hover over the section just to the left, the white with hashmarks, you see this:
That’s 4.64 GB of purgeable space. What exactly is purgeable space? It’s anything that Apple thinks you can remove. This may include files in the trash, or storage that can be “optimized” in iTunes.
In my case, it just so happens that I have 4.42 GB of TV shows in my iTunes library:
There’s a small difference between that amount and the total purgeable amount of my MacBook.
That’s nearly the amount of purgeable space on my drive. If I were to delete those files, then I would have almost the amount of free space that the Finder shows.
So that free space you think you’ve recovered? You haven’t. It’s potential free space that you could recover if you follow all of Apple’s recommendations. This does not include potential savings of storing your Desktop and Documents folders in the cloud. On my iMac, where my Documents folder is about 8 GB, I only see about 2 GB of purgeable space. So it’s not clear what this space consists of. But if you seem to have a lot of free space, it’s most likely because of movies or TV shows from the iTunes Store that Apple thinks you can re-download.
It’s worth noting that my iMac tells me that I can save more than 12 GB by removing instruments and lessons from Garage Band.
Yet Garage Band only uses about 2 GB for those files, and if I click Open Garage Band, the app opens, but I see no way to reclaim that space.